U.S. patent application number 11/235158 was filed with the patent office on 2006-01-26 for optical waveguide and method of manufacture.
This patent application is currently assigned to Nippon Telegraph and Telephone Corporation. Invention is credited to Makoto Abe, Koji Enbutsu, Kazuo Fujiura, Tadayuki Imai, Eishi Kubota, Takashi Kurihara, Masahiro Sasaura, Seiji Toyoda, Shogo Yagi.
Application Number | 20060018617 11/235158 |
Document ID | / |
Family ID | 27482268 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060018617 |
Kind Code |
A1 |
Sasaura; Masahiro ; et
al. |
January 26, 2006 |
Optical waveguide and method of manufacture
Abstract
An optical waveguide capable of having various characteristics
and a method of manufacture thereof as well as a method of
manufacturing a crystal film are provided. An optical functional
material KTa.sub.xNb.sub.1-xO.sub.3 is used as an optical
waveguide. The input optical signal is transmitted to the
KTa.sub.xNb.sub.1-xO.sub.3 film. The KTa.sub.xNb.sub.1-xO.sub.3
film undergoes changes in optical property when an external voltage
signal-is applied to the electrode. Therefore, as it passes through
the KTa.sub.xNb.sub.1-xO.sub.3 film, the input optical signal is
modulated by the characteristic change. The modulated optical
signal is taken out as an output optical signal.
Inventors: |
Sasaura; Masahiro; (Ibaraki,
JP) ; Fujiura; Kazuo; (Ibaraki, JP) ; Enbutsu;
Koji; (Ibaraki, JP) ; Imai; Tadayuki;
(Ibaraki, JP) ; Yagi; Shogo; (Ibaraki, JP)
; Kurihara; Takashi; (Ibaraki, JP) ; Abe;
Makoto; (Ibaraki, JP) ; Toyoda; Seiji;
(Ibaraki, JP) ; Kubota; Eishi; (Ibaraki,
JP) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20045-9998
US
|
Assignee: |
Nippon Telegraph and Telephone
Corporation
Tokyo
JP
|
Family ID: |
27482268 |
Appl. No.: |
11/235158 |
Filed: |
September 27, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10909433 |
Aug 3, 2004 |
|
|
|
11235158 |
Sep 27, 2005 |
|
|
|
10142964 |
May 13, 2002 |
6792189 |
|
|
10909433 |
Aug 3, 2004 |
|
|
|
Current U.S.
Class: |
385/129 |
Current CPC
Class: |
Y10T 29/49016 20150115;
G02B 2006/12176 20130101; G02B 2006/12035 20130101; G02F 1/035
20130101; G02F 1/3551 20130101; G02B 2006/121 20130101; G02B
2006/12164 20130101; G02B 2006/12097 20130101; G02F 1/0027
20130101; G02B 2006/12142 20130101; G02B 6/13 20130101; Y10S
117/918 20130101; G02B 2006/12109 20130101; G02B 2006/12173
20130101; G02B 2006/12178 20130101; G02B 6/132 20130101; G02B
2006/1218 20130101 |
Class at
Publication: |
385/129 |
International
Class: |
G02B 6/10 20060101
G02B006/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 2001 |
JP |
2001-143570 |
May 14, 2001 |
JP |
2001-143571 |
May 16, 2001 |
JP |
2001-146560 |
May 31, 2001 |
JP |
2001-165283 |
Claims
1-23. (canceled)
24. A method of manufacturing a crystal film having a composition
of KTa.sub.1-xNb.sub.xO.sub.3 (0<x<1), the method comprising
steps of: introducing in the form of gas flows into a reaction
system having a substrate .beta.-diketone complex of K (R is an
alkyl group with a carbon number of 1 to 7, R' is an alkyl group or
C.sub.nF.sub.2n+1, and n is 1 to 3) expressed by a general formula
(1) as a first initial material component, ##STR3## at least one of
a gaseous Ta compound and a volatile Ta compound as a second
initial material component, at least one of a gaseous Nb compound
and a volatile Nb compound as a third initial material component,
and an oxygen-containing gas used as an oxidizer, and reacting
these components in a gas phase or on the substrate to form a
crystal of KTa.sub.1-xNb.sub.xO.sub.3 on the substrate.
25. A method of manufacturing a crystal film as claimed in claim
24, wherein the oxygen-containing gas is oxygen or a mixture gas of
oxygen and at least one of hydrogen and nitrogen.
26. A method of manufacturing a crystal film as claimed in claim
25, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
27. A method of manufacturing a crystal film as claimed in claim
24, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
28. A method of manufacturing a crystal film having a composition
of K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 (0<x<1 and
0<y<1), the method comprising steps of: introducing in the
form of gas flows into a reaction system having a substrate
.beta.-diketone complex of K (R is an alkyl group with a carbon
number of 1 to 7, R' is an alkyl group or C.sub.nF.sub.2n+1, and n
is 1 to 3) expressed by a general formula (1) as a first initial
material component, ##STR4## .beta.-diketone complex of Li (R is an
alkyl group with a carbon number of 1 to 7, R' is an alkyl group or
C.sub.nF.sub.2n+1, and n is 1 to 3) expressed by a general formula
(2) as a second initial material component, ##STR5## at least one
of a gaseous Ta compound and a volatile Ta compound as a third
initial material component, at least one of a gaseous Nb compound
and a volatile Nb compound as a fourth initial material component,
and an oxygen-containing gas used as an oxidizer, and reacting
these components in a gas phase or on the substrate to form a
crystal of KTa.sub.1-xNb.sub.xO.sub.3 on the substrate.
29. A method of manufacturing a crystal film as claimed in claim
28, wherein the oxygen-containing gas is oxygen or a mixture gas of
oxygen and at least one of hydrogen and nitrogen.
30. A method of manufacturing a crystal film as claimed in claim
29, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
31. A method of manufacturing a crystal film as claimed in claim
28, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
32. A method of manufacturing a crystal film having a composition
of KTa.sub.1-xNb.sub.xO.sub.3 (0<x<1), the method comprising
steps of: introducing in the form of gas flows into a reaction
system having a substrate .beta.-diketone complex of K (R is an
alkyl group with a carbon number of 1 to 7, R' is an alkyl group or
C.sub.nF.sub.2n+1, and n is 1 to 3) expressed by a general formula
(1) as a first initial material component, ##STR6## at least one of
a gaseous Ta compound and a volatile Ta compound as a second
initial material component, and at least one of a gaseous Nb
compound and a volatile Nb compound as a third initial material
component, and reacting these components in a gas phase or on the
substrate to form a crystal of KTa.sub.1-xNb.sub.xO.sub.3 on the
substrate.
33. A method of manufacturing a crystal film as claimed in claim
32, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
34. A method of manufacturing a crystal film having a composition
of K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 (0<x<1 and
0<y<1), the method comprising steps of: introducing in the
form of gas flows into a reaction system having a substrate
.beta.-diketone complex of K (R is an alkyl group with a carbon
number of 1 to 7, R' is an alkyl group or C.sub.nF.sub.2n+1, and n
is 1 to 3) expressed by a general formula (1) as a first initial
material component, ##STR7## .beta.-diketone complex of Li (R is an
alkyl group with a carbon number of 1 to 7, R' is an alkyl group or
C.sub.nF.sub.2n+1, and n is 1 to 3) expressed by a general formula
(2) as a second initial material component, ##STR8## at least one
of a gaseous Ta compound and a volatile Ta compound as a third
initial material component, and at least one of a gaseous Nb
compound and a volatile Nb compound as a fourth initial material
component, and reacting these components in a gas phase or on the
substrate to form a crystal of KTa.sub.1-xNb.sub.xO.sub.3 on the
substrate.
35. A method of manufacturing a crystal film as claimed in claim
34, wherein an energy necessary for the reaction is supplied from
at least one of heat, light and plasma.
Description
[0001] This application is based on Japanese Patent Application
Nos. 2001-143570 filed May 14, 2001, 2001-143571 filed May 14,
2001, 2001-146560 filed May 16, 2001 and 2001-165283 filed May 31,
2001, the contents of which are incorporated hereinto by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical waveguide and a
method of manufacture thereof and more particularly to an optical
waveguide for novel functional optical integrated circuits using an
optical functional material KTa.sub.xNb.sub.1-xO.sub.3 as the
optical waveguide and a method of manufacture thereof and also to a
method of manufacturing a crystal film for use with optical
communication devices.
[0004] 2. Description of the Related Art
[0005] Intensive research and development efforts are being made
from a cost performance point of view to develop optical integrated
circuits that integrate on a single substrate optical devices that
perform emission, detection, modulation, and multiplexing and
demultiplexing of light. This integration technology is expected to
reduce electric power, enhance performance and reduce cost of these
optical devices.
[0006] Conventional optical integrated circuits currently in wide
use have a fabrication in which a waveguide structure is formed on
a semiconductor substrate using SiO.sub.2 and polymers to process
an optical signal launched from outside. The waveguide structure
refers to a structure comprising an undercladding layer, a
waveguide layer formed on the undercladding layer and having a
refractive index higher than that of the undercladding layer, and
an overcladding layer covering the waveguide layer and having a
refractive index smaller than that of the waveguide layer. To
realize a function of optical signal processing, the conventional
optical ICs change an optical properties of the waveguide material
as represented by ordinary and extraordinary refractive indices by
applying external fields, such as heat, electric fields, magnetic
fields and sound, thereby achieving such functions as
multiplexing/demultiplexing optical signals and adjusting a
transfer time.
[0007] However, since the waveguide materials currently available
are limited to SiO.sub.2, polymers, semiconductors and a small
range of nonlinear crystals, the changing of the optical properties
as realized by the method described above is greatly restricted by
the characteristic of the waveguide material used, thus imposing
limitations on the applicable optical signal processing.
[0008] Under these circumstances, the use of a novel waveguide
material KTa.sub.xNb.sub.1-xO.sub.3 is being considered. The
optical functional material KTa.sub.xNb.sub.1-xO.sub.3 exhibits an
optical second-order nonlinear effect. An optical nonlinear
constant of this material is 1,200-8,000 pm/V, significantly larger
than 31 pm/V which is an optical nonlinear constant of LiNbO.sub.3
for example.
[0009] Further, since this optical nonlinear effect is attributed
to the displacement of positions of constitutional elements by the
application of an electric field, the presence or absence of the
optical nonlinear effect can be controlled by the application of an
electric field.
[0010] The material KTa.sub.xNb.sub.1-xO.sub.3 undergoes a
ferroelectric phase transition at a composition-dependent Curie
temperature of between -250.degree. C. and 400.degree. C. At this
Curie temperature as a boundary the material's property changes
significantly. For example, its dielectric constant greatly changes
from approximately 3,000 to about 20,000. It is possible to create
a new optical integrated circuit taking advantage of the
ferroelectric phase transition. The Curie temperature varies
depending on the composition x of KTa.sub.xNb.sub.1-xO.sub.3, and
adding Li to KTa.sub.xNb.sub.1-xO.sub.3 to produce
K.sub.yLi.sub.1-yTa.sub.xNb.sub.1-xO.sub.3 makes it possible to
adjust the temperature range.
[0011] The fabrication process of an optical waveguide requires
steps of first forming a waveguide material film and then
performing patterning and etching on the film using
photolithography or the like.
[0012] The currently used waveguide materials, however, are limited
to SiO.sub.2, polymers, semiconductors and a small range of
nonlinear crystals. Hence, the modification of optical properties
as realized by the aforementioned application of heat, electric
fields, magnetic fields or sound is greatly restricted by the
characteristics of the waveguide material used. The conventional
optical ICs therefore have a problem that the applicable range of
optical signal processing is very narrow.
[0013] Further, the method of manufacturing an optical waveguide
using the KTa.sub.xNb.sub.1-xO.sub.3 optical functional material
described above also requires the fabrication process, similar to
the conventional one, of forming a film of the waveguide material
and patterning the waveguide film by photolithography. Therefore,
even in using the novel waveguide material
KTa.sub.xNb.sub.1-xO.sub.3, the conventional technology has a
problem that the waveguide fabrication process is complex.
[0014] Another problem is that, although the waveguide fabrication
is essential in obtaining a desired performance, a technique to
form waveguides in a KTN crystal has not yet been established. This
is attributed to the fact that ions that increase the refractive
index and still do not degrade the nonlinear characteristic after
diffusion has not been found.
[0015] The chemical vapor deposition (CVD) method vaporizes a
material containing constitutional components and causes a desired
reaction in a gas phase or on a substrate. Forming a waveguide
material film by using the CVD method requires a volatile compound
containing the constitutional components. In KTN or KLTN, as to the
compounds of Ta and Nb, halide and alkoxide have high volatility
and can be used as the starting material when the CVD method is
applied.
[0016] As to K and Li compounds, there is not much information
available about the materials which provide sufficient vapor
pressures. In the case of K in particular, no material has been
known which is effective for use with the CVD method. The essential
reason for this is that alkali metal elements such as K and Li tend
to be ionized easily and cannot easily be kept in a molecular state
necessary for vaporization.
SUMMARY OF THE INVENTION
[0017] The present invention has been accomplished to overcome
these problems and provide an optical waveguide and a method of
manufacture thereof, the optical waveguide being capable of having
a variety of characteristics not achievable with conventional
devices and of forming a waveguide easily.
[0018] Another object of this invention is to provide a diffused
waveguide and a method of manufacture thereof, the diffused
waveguide allowing a KTN crystal to be formed into a waveguide by
diffusing Li, a technique not achievable with conventional
devices.
[0019] To achieve these objective, the present invention provides
an optical waveguide comprising: an undercladding layer; a
waveguide layer formed on the undercladding layer and having a
higher refractive index than that of the undercladding layer; and
an overcladding layer covering the waveguide layer and having a
lower refractive index than that of the waveguide layer; wherein
the undercladding layer is a substrate and the waveguide layer is
formed from an optical functional material
KTa.sub.xNb.sub.1-xO.sub.3 (0<x<1).
[0020] Further, the substrate is one of a
KTa.sub.yNb.sub.1-yO.sub.3 (O.ltoreq.y.ltoreq.1, y.noteq.x)
substrate, a MgO substrate, a MgAl.sub.2O.sub.4 substrate and a
NdGaO.sub.3 substrate.
[0021] Further, the undercladding layer comprises the substrate and
one of SiO.sub.2, KTa.sub.zNb.sub.1-xO.sub.3 (0.ltoreq.z.ltoreq.1,
z.noteq.x), MgO, MgAl.sub.2O.sub.4 and NdGaO.sub.3 deposited on the
substrate.
[0022] Further, the overcladding layer is formed from one of
KTa.sub.uNb.sub.1-uO.sub.3 (0.ltoreq.u.ltoreq.1, u.noteq.x), MgO,
MgAl.sub.2O.sub.4, NdGaO.sub.3 and polymer.
[0023] Further, an optical waveguide comprises: an undercladding
layer; a waveguide layer formed on the undercladding layer and
having a higher refractive index than that of the undercladding
layer; and an overcladding layer covering the waveguide layer and
having a lower refractive index than that of the waveguide layer;
wherein the undercladding layer is a substrate and the waveguide
layer is formed from an optical functional material
K.sub.1-vLi.sub.vTa.sub.xNb.sub.1-xO.sub.3 (0<x<1,
0<v.ltoreq.0.5).
[0024] This invention is characterized in that the optical
waveguide is formed from an optical functional material
KTa.sub.xNb.sub.1-xO.sub.3 whose optical properties represented by
an electrooptical effect (EO effect), an acoustooptic effect (AO
effect) and a figure of merit are remarkably large when compared
with those of conventional waveguide materials.
[0025] The optical functional material KTa.sub.xNb.sub.1-xO.sub.3
is a paraelectric crystal material and has a cubic structure with a
refractive index of 2.4 at temperature higher than ferroelectric
transition. When an external field is applied in the crystal axis
direction, the resulting positional displacement of the
constitutional elements produces an optical second-order nonlinear
effect. The optical nonlinearity constant of this optical
functional material is 1,200-8,000 pm/V, significantly larger than,
for example, 31 pm/V which is the optical nonlinearity constant of
LiNbO.sub.3.
[0026] The optical nonlinear effect is the result of the positional
displacement of constitutional elements caused by the application
of an electric field. Hence, the presence or absence of the optical
nonlinear effect can be controlled by the application of an
electric field. The material KTa.sub.xNb.sub.1-xO.sub.3 undergoes a
ferroelectric phase transition at a composition-dependent Curie
temperature of between -250.degree. C. and 400.degree. C. At this
Curie temperature as a boundary the material's property changes
sharply. For example, its dielectric constant greatly changes from
approximately 3,000 to about 20,000. It is therefore possible to
create a new optical integrated circuit taking advantage of optical
characteristic changes caused by the ferroelectric phase
transition.
[0027] The Curie temperature varies depending on the composition x
of KTa.sub.xNb.sub.1-xO.sub.3, and adding Li to
KTa.sub.xNb.sub.1-xO.sub.3 can adjust its Curie temperature
range.
[0028] Further, this invention provides a method of manufacturing
an optical waveguide, wherein the optical waveguide comprises an
undercladding layer, a waveguide layer formed on the undercladding
layer and having a higher refractive index than that of the
undercladding layer, and an overcladding layer covering the
waveguide layer and having a lower refractive index than that of
the waveguide layer, the method comprising steps of: using the
undercladding layer as a substrate and forming on the substrate a
structure constituting a crystal growth nucleation position; and
growing a thin film of an optical functional material
KTa.sub.xNb.sub.1-xO.sub.3 (0<x<1) into a rectangular
parallelepiped with the structure as a center to form the waveguide
layer.
[0029] Further, this invention provides a method of manufacturing
an optical waveguide, wherein the optical waveguide comprises an
undercladding layer, a waveguide layer formed on the undercladding
layer and having a-higher refractive index than that of the
undercladding layer, and an overcladding layer covering the
waveguide layer and having a lower refractive index than that of
the waveguide layer, the method comprising the steps of: using the
undercladding layer as a substrate and forming on the substrate a
structure constituting a crystal growth nucleation position; and
growing a thin film of an optical functional material
K.sub.1-yLi.sub.yTa.sub.xNb.sub.1-xO.sub.3 (0<x<1,
0<y.ltoreq.0.5) into a rectangular parallelepiped with the
structure as a center to form the waveguide layer.
[0030] An ordinary waveguide fabrication process involves
depositing a film of the material for a waveguide layer over a
large area and then patterning the film into a desired
configuration of the waveguide rectangular in cross section by
photolithography. This invention takes advantage of the fact that
the waveguide material is KTa.sub.xNb.sub.1-xO.sub.3 crystal and,
instead of the ordinary process described above, forms the optical
waveguide rectangular in cross section in a single film making
step.
[0031] The optical waveguide fabrication method of this invention
requires depositing a thin film of KTa.sub.xNb.sub.1-xO.sub.3 with
optical characteristics sufficient for light propagation, i.e.,
satisfactory crystal quality that produces such characteristics,
and then forming the film into a predetermined structure at a
predetermined location according to a design of the optical
integrated circuit. Such an optical quality can be realized by a
crystal epitaxial growth method. In a field of semiconductor
crystal growth technology, an epitaxial growth method available
that grows thin films having a high degree of lattice mismatch
between a substrate and a thin film, as in the case of
GaN-on-sapphire and GaAs-on-Si, is a micro-channel epitaxy (for
example, T. Nishinaga and H. J. Scheel, "Advances in
Superconductivity VIII," ed. By H. Hayakawa and Y. Enomoto
(Springer-Verlag, Tokyo, 1996) p. 33). This micro-channel epitaxy
controls the thin film growth nucleation position by a groove
formed on the upper surface of a seed layer on the substrate and
improves the crystal quality of the thin film by the horizontal
growth of the thin film from the nucleation position.
[0032] In this invention, since the nucleation position and the
thin film growth direction can be controlled, when the crystal
material has a strong crystal habit, it is possible to create a
structure enclosed by the singular faces of the crystal material.
The KTa.sub.xNb.sub.1-xO.sub.3 crystal material used in this
invention has a cubic crystal structure and a strong crystal habit
which is constructed by the {100} singular faces, so that a
rectangular thin film enclosed by the {100} planes is likely to
grow. In the process of growing a thin film, the growth nuclei on
the substrate are generated starting from where the surface energy
of the substrate is smallest. When there are holes or grooves on a
planar substrate, the areas of the holes or grooves have side
surfaces in addition to the bottom surfaces, increasing the number
of contact surfaces with which the material supplied onto the
substrate comes into contact. It is apparent also from the
classical theory of crystal growth that an increase in the number
of contact surfaces lowers the surface energy of the areas of the
holes or grooves and thus the probability of crystal nuclei being
generated in these areas becomes higher than in other planar
areas.
[0033] Therefore, by forming in advance holes or grooves in that
substrate contact surface where a rectangular waveguide is to be
formed, the KTa.sub.xNb.sub.1-xO.sub.3 crystal material can be made
to start growing a thin film at the holes or grooves as the growth
nucleation points and fill these holes or grooves. If the growth of
the KTa.sub.xNb.sub.1-xO.sub.3 crystal material is continued, a
growth in the horizontal direction of the substrate, i.e., along
the free surface, also starts, in addition to the growth in the
vertical direction of the substrate at the holes or grooves. At
this time, as to the growth in the horizontal direction of the
substrate, the film being grown is limited in shape by the {100}
singular faces of the KTa.sub.xNb.sub.1-xO.sub.3 crystal material.
Thus, a film of KTa.sub.xNb.sub.1-xO.sub.3 having a rectangular
parallelepiped structure enclosed by {100} planes can be
produced.
[0034] Further, this invention provides a diffused waveguide formed
by diffusing ions in a crystal and using as a waveguide core an
area of the crystal diffused with the ions and having a higher
refractive index than those of surrounding areas, wherein the
crystal has a composition of KTa.sub.1-xNb.sub.xO.sub.3 and the
ions are Li.
[0035] Further, this invention forms a waveguide core with a higher
refractive index than those of the surrounding areas by diffusing
Li ions in the crystal of a composition of
KTa.sub.1-xNb.sub.xO.sub.3.
[0036] That is, this invention is characterized by Li ions being
diffused in the KTN crystal to form a core with a high refractive
index. Li ions can be thermally diffused by substituting a K site
and the KLTN crystal having the composition of
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 also has a performance
equal to or higher than that of the KTN crystal. Therefore, there
is no possibility of characteristic degradation due to ion
diffusion. Further, the relative index difference obtained by
adding Li through thermal diffusion is 2% or higher, which is
sufficient for the fabrication of a waveguide. This means that Li
is an appropriate ion for forming waveguide. The melt containing
LiNO.sub.3 used for diffusion has a low melting point of
261.degree. C., which means that a stable melt can be obtained
easily. This melt has a high water solubility so that, after the
diffusion processing, it can be easily washed away with water.
Thus, it has no adverse effects on the subsequent thermal diffusion
processing in a gas.
[0037] As described above, the manufacture of a diffused waveguide
using Li ions has-advantages that it can control the refractive
index without degrading its characteristics and that the diffusion
process using LiNO.sub.3 is simple and can perform diffusion at low
temperatures.
[0038] Further, this invention provides a method of manufacturing a
crystal film having a composition of KTa.sub.1-xNb.sub.xO.sub.3
(0<x<1), the method comprising steps of: introducing, in the
form of gas flows into a reaction system having a substrate,
.beta.-diketone complex of K (R is an alkyl group with a carbon
number of 1 to 7, R' is an alkyl group or C.sub.nF.sub.2n+1, and n
is 1 to 3) expressed by a general formula (1) as a first initial
material component, ##STR1## at least one of a gaseous Ta compound
and a volatile Ta compound as a second initial material component,
at least one of a gaseous Nb compound and a volatile Nb compound as
a third initial material component, and an oxygen-containing gas
used as an oxidizer, and reacting these components in a gas phase
or on the substrate to form a crystal of KTa.sub.1-xNb.sub.xO.sub.3
on the substrate.
[0039] The above and other objects, effects, features and
advantages of the present invention will become more apparent from
the following description of embodiments thereof taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a perspective view showing an example fabrication
of an optical waveguide applying the present invention;
[0041] FIG. 2 is a perspective view showing another example
fabrication of an optical waveguide applying the present
invention;
[0042] FIG. 3 is a perspective view showing a substrate formed with
a groove as a crystal growth nucleation position for a
KTa.sub.xNb.sub.1-xO.sub.3 crystal;
[0043] FIG. 4 is a perspective view showing a substrate formed with
holes as crystal growth nucleation positions for
KTa.sub.xNb.sub.1-xO.sub.3 crystals;
[0044] FIG. 5 is a perspective view showing a substrate having an
electrode layer formed with holes as crystal growth nucleation
positions for KTa.sub.xNb.sub.1-xO.sub.3 crystals;
[0045] FIG. 6 is a cross-sectional view taken along the line VI-VI
of FIG. 5;
[0046] FIG. 7 is a graph showing a SIMS analysis result of a Li ion
distribution in Embodiment 3-1;
[0047] FIG. 8 is a graph showing a SIMS analysis result of a Li ion
distribution after an internal diffusion in Embodiment 3-1;
[0048] FIG. 9 is a graph showing a refractive index distribution
after an internal diffusion of Li ions in. Embodiment 3-1;
[0049] FIG. 10 illustrates a cross-sectional view of a waveguide
after a rediffusion of K, and a diagram showing a SIMS analysis
result of a Li ion distribution in Embodiment 3-2;
[0050] FIG. 11 is a perspective view showing a fabrication of a
wavelength conversion device in Embodiment 3-3;
[0051] FIG. 12 is a diagram showing a wavelength conversion
spectrum in Embodiment 3-4 and Embodiment 4-14;
[0052] FIG. 13 is a schematic diagram of a film deposition
apparatus in Embodiment 4-1;
[0053] FIG. 14 is a graph showing a measurement result of a
dielectric constant of a KTN film fabricated in Embodiment 4-1;
[0054] FIG. 15 is a graph showing a measurement result of a
dielectric constant of a KLTN film fabricated in Embodiment
4-13;
[0055] FIG. 16A is a perspective view of a wavelength conversion
device fabricated in Embodiment 4-14; and
[0056] FIG. 16B is a cross-sectional view of the wavelength
conversion device taken along the line XVIB-XVIB of FIG. 16A.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0057] Embodiments of the present-invention will be described in
detail. In realizing an optical waveguide of this invention it is
necessary to deposit a thin film of KTa.sub.xNb.sub.1-xO.sub.3
having an satisfactory optical characteristic or crystal quality
for lightwave propagation and form it into a desired structure at a
predetermined location according to a design of an optical
integrated circuit. The process of manufacturing an optical
waveguide according to the present invention can be classed largely
into four basic processes:
[0058] 1) Forming a KTa.sub.xNb.sub.1-xO.sub.3 thin film on a
substrate;
[0059] 2) Manufacturing a waveguide layer by processing the
KTa.sub.xNb.sub.1-xO.sub.3 thin film;
[0060] 3) Manufacturing a mechanism for applying external fields to
the KTa.sub.xNb.sub.1-xO.sub.3 waveguide layer; and
[0061] 4) Manufacturing a cover layer over the
KTa.sub.xNb.sub.1-xO.sub.3 waveguide layer.
[0062] These four manufacturing processes are referred to as a
manufacturing process 1, a manufacturing process 2, a manufacturing
process 3 and a manufacturing process 4 respectively and will be
explained in the following.
[0063] The order of these four manufacturing processes is
determined according to a design structure of the optical
integrated circuit. In addition, the determination of this order
takes into account whether a material region already fabricated by
one manufacturing process may or may not be degraded in quality by
the subsequent manufacturing process. Hence, it should be noted
that the actual order of the manufacturing processes does not
necessarily agree with the order indicated by the accompanying
numbers.
[0064] Since the waveguide layer material is
KTa.sub.xNb.sub.1-xO.sub.3, the materials forming the substrate and
the cover layer need to have a sufficient refractive index
difference with respect to the refractive index of
KTa.sub.xNb.sub.1-xO.sub.3 to confine light in the waveguide
layer.
Embodiment 1
[0065] FIG. 1 is a perspective view showing the fabrication of an
optical waveguide applying the present invention. A basic
fabrication shown here as an example waveguide structure for
explanation uses a ridge type optical waveguide as an optical
waveguide and an electric field application from an electrode as an
external field application mechanism. In the figure, reference
number 1 represents a substrate that functions as an undercladding
layer, 2 a waveguide layer having a refractive index higher than
that of the substrate 1 and formed from a film of an optical
functional material KTa.sub.xNb.sub.1-xO.sub.3 (0<x<1), 3 an
input optical signal, 4 an electrode, 5 an output optical signal,
and 6 an overcladding layer (cover layer) covering the waveguide
layer and having a refractive index lower than that of the
waveguide layer KTa.sub.xNb.sub.1-xO.sub.3 film 2.
[0066] The input optical signal 3 is transmitted into the
KTa.sub.xNb.sub.1-xO.sub.3 film 2. The KTa.sub.xNb.sub.1-xO.sub.3
film 2 changes its optical property by an external voltage signal
applied to the electrode 4. Thus, the optical signal is modulated
by the film's optical property change as it passes through the
KTa.sub.xNb.sub.1-xO.sub.3 film 2. The modulated optical signal is
taken out as the output optical signal 5.
[0067] The waveguide layer may use as an alternative an optical
functional material K.sub.1-xLi.sub.vTa.sub.xNb.sub.1-xO.sub.3
(1<x<1, 0<v.ltoreq.0.5).
[0068] Although in the following embodiments only the manufacturing
methods we tested will be described, it is quite obvious to persons
skilled in the art that, for the individual material region
manufacturing processes classified earlier, known thin film
manufacturing techniques, such as liquid phase epitaxy, physical
deposition, chemical vapor deposition and sol-gel processing, can
be applied and that the known masking and etching techniques can be
applied to the forming of individual structures.
Embodiment 1-1
[0069] First, the manufacturing process 1 will be explained. In
this embodiment, the liquid phase epitaxy capable of forming a
single crystal film with high quality was adopted as the method of
depositing a high quality KTa.sub.xNb.sub.1-xO.sub.3 film on the
substrate 1. A KTa.sub.xNb.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1)
substrate, a MgO substrate, a MgAl.sub.2O.sub.4 substrate and a
NdGaO.sub.3 substrate were used for the substrate 1. As the buffer
layer (undercladding layer) were used a semiconductor Si substrate,
a GaAs substrate and an InP substrate, which were formed by
depositing such materials as SiO.sub.2, KTa.sub.xNb.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1), MgO, MgAl.sub.2O.sub.4 and NdGaO.sub.3.
[0070] To prevent a possible contamination, the
KTa.sub.xNb.sub.1-xO.sub.3 film 2 was formed by using potassium
oxide and KTa.sub.xNb.sub.1-xO.sub.3 oxide as a solvent and a
solute, respectively, and by using a self-flux method. As the
potassium oxide solvent, a mixture of KVO.sub.3 and K.sub.2O
(KVO.sub.3=30-70 mol %) or K.sub.2CO.sub.3 carbonate was used. When
a KTa.sub.yNb.sub.1-yO.sub.3 (0.ltoreq.y.ltoreq.1) substrate was
used, a material with x (0<x<1) composition different from
the substrate composition was selected so that there was a
difference in refractive index between the substrate and the
waveguide layer. Hence, in the following description the material
used as the waveguide layer and the material used as the substrate
1 are shown to have compositions of KTa.sub.xNb.sub.1-xO.sub.3
(0<x<1) and KTa.sub.yNb.sub.1-yO.sub.3 (0.ltoreq.y.ltoreq.1),
respectively.
[0071] To change the characteristic of the waveguide layer, 0-10
mol % of LiCO.sub.3 was added to KTa.sub.xNb.sub.1-xO.sub.3
(0<x<1). The solute concentration with respect to the solvent
was set to 30-50 mol %, a concentration range where the object
material KTa.sub.xNb.sub.1-xO.sub.3 is shown in a phase diagram to
precipitate as an initial crystal. The film deposition temperature
was set at 0-10.degree. C. supercooled from a solid-solution
equilibrium temperature of 1,050-1,360.degree. C. determined from
the liquidus of the phase diagram and the concentration of the
solute used. This temperature was used considering the fact that a
low film deposition rate constitutes one of conditions for forming
a high quality film. However, there was also a case where a growth
rate with a larger supercooling had to be used when considering a
lattice matching between the KTa.sub.xNb.sub.1-xO.sub.3 and the
substrate material used and a reactivity between the solution and
the substrate used.
[0072] The film deposition rate achieved was around 1 .mu.m/min.
The film deposition time was determined from the precalculated
deposition rate and the desired film thickness.
[0073] The crystal quality of the KTa.sub.xNb.sub.1-x0.sub.3 film
deposited to a thickness of 2 .mu.m was best when the substrate
used the same material as the film material, i.e., the material
that satisfies the homo epitaxial condition. The
KTa.sub.xNb.sub.1-xO.sub.3 film thus formed has an optical quality
of 0.10 dB/cm in terms of optical transmission loss. The
KTa.sub.xNb.sub.1-xO.sub.3 film was verified by the X-ray
diffraction method to be oriented in a <100> axis
direction.
Embodiment 1-2
[0074] Next, the manufacturing process 2 will be explained.
Photolithography was used to process the KTa.sub.xNb.sub.1-xO.sub.3
film to form a waveguide layer of a ridge type optical waveguide. A
resist was applied to the thin film thus formed and was exposed and
developed by using a mask for a 2 .mu.m wide waveguide. The
material was then etched by an ion milling method and the residual
resist was removed, thereby forming a waveguide layer of the ridge
type waveguide. The waveguide layer is nearly a 2 .mu.m square and
an observation of the side wall using a SEM (scanning electron
microscope) found no significantly roughened surface.
[0075] The measurement of loss of this ridge type waveguide at a
wavelength of 1.55 .mu.m showed a loss value of 0.99 dB/cm, which
means that the waveguide layer formed is good enough so that
undulations of the surface as a result of processing can be
neglected.
Embodiment 1-3
[0076] Next, the manufacturing process 3 will be explained. The
electrode material is preferably a metal or conductive oxide highly
chemically stable with the KTa.sub.xNb.sub.1-xO.sub.3 film. In this
embodiment, we will describe a result of manufacture with Au used
as an electrode material.
[0077] In manufacturing an Au electrode of a shape according to the
design, techniques commonly used in the semiconductor integrated
circuit manufacturing process were applied. More specifically, the
Au thin film deposited by sputtering was processed by
photolithography.
[0078] The techniques used in this manufacturing process 3 are
common and therefore their explanations are omitted.
Embodiment 1-4
[0079] The material used for the cover layer 6 was chosen from
among such oxides as KTa.sub.uNb.sub.1-uO.sub.3
(0.ltoreq.u.ltoreq.1), MgO, MgAl.sub.2O.sub.4 and NdGaO.sub.3, and
a polymer that retards degradation of the already fabricated
waveguide and electrode structure. In this embodiment, we will
describe a result of fabrication obtained when a sputtering method
with a low substrate temperature during evaporation was used to
deposit KNbO.sub.3.
[0080] For the fabrication of the cover layer 6 an opposed type ion
sputtering apparatus was used. Nb and K.sub.2CO.sub.3 targets were
mounted on one surface and, on the opposing surface 2-10 cm from
the substrate, was mounted. As an ambient gas a Xe gas was used at
a pressure of 1.times.10.sup.-4 to 3.times.10.sup.-4 Torr. A
voltage was applied between the targets and the substrate to
produce an energy of 400-800 eV for Nb ions and K.sub.2CO.sub.3
ions to deposit a film of KNbO.sub.3 cover layer at the substrate
temperature of 500-700.degree. C. The film was deposited to a
thickness of 0.1 .mu.m at a rate of 3 .ANG./min. The thin film thus
obtained was verified to be KNbO.sub.3 by the X-ray diffraction
method.
[0081] The same technique as used in the manufacturing process 2
was used to open holes in the upper surface of the electrode 4 to
lead conductive wires into the electrode 4 formed by the
manufacturing process 3.
[0082] The basic optical integrated circuit fabricated by the
manufacturing process 1, manufacturing process 2, manufacturing
process 3 and manufacturing process 4 in that order was cut and its
cross section was observed with a SEM, which revealed no
significant structural deterioration. Further, the performance of
the electrode 4 connected with conductive wires was evaluated using
the input optical signal 3 with a wavelength of 1.55 .mu.m. It was
confirmed that the wavelength conversion device and the modulator
had high performances, i.e., high efficiency and low noise, as
expected from the design.
[0083] We have described a case where the waveguide layer of
KTa.sub.xNb.sub.1-xO.sub.3 (0<x<1) was formed on a substrate.
In a general core-cladding structure, optical waveguides can of
course be provided with a variety of characteristics, not possible
with the conventional techniques, by forming the core from
KTa.sub.xNb.sub.1-xO.sub.3 (0<x<1).
Embodiment 2
[0084] FIG. 2 is a perspective view showing another fabrication of
an optical waveguide applying the present invention. In this case,
the waveguide structure is a rectangular parallelepiped extending
along the substrate. In the following, the rectangular
parallelepiped structure film is described to be fabricated by a
thin film growth method. In the figure, reference numeral 11
represents a substrate, 12 a KTa.sub.xNb.sub.1-xO.sub.3 film, 13 an
input optical signal, 14 an electrode, 15 an output optical signal,
and 16 a groove. The input optical signal 13 is transmitted into
the KTa.sub.xNb.sub.1-xO.sub.3 film 12. The
KTa.sub.xNb.sub.1-xO.sub.3 film 12 changes its optical property by
an external voltage signal applied to the electrode 4. Thus, the
optical signal is modulated by the film's optical property change
as it passes through the KTa.sub.xNb.sub.1-xO.sub.3 film 12. The
modulated optical signal is taken out as the output optical signal
15.
[0085] To deposit a high quality KTa.sub.xNb.sub.1-xO.sub.3 film on
the substrate, a liquid phase epitaxy capable of forming a high
quality single crystal film was selected as a film deposition
method. The substrate material was chosen by considering mainly a
lattice matching, a basic condition for the epitaxial growth. The
substrate was chosen from among a KTa.sub.xNb.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1) substrate, a MgO substrate, a MgAl.sub.2
substrate and a NdGaO.sub.3 substrate.
[0086] Further, a semiconductor Si substrate, GaAs substrate or InP
substrate deposited with SiO.sub.2, KTa.sub.xNb.sub.1-xO.sub.3
(0.ltoreq.x.ltoreq.1), MgO, MgAl.sub.2O.sub.4 or NdGaO.sub.3 was
also used. A photolithography method commonly used in the
fabrication of semiconductor integrated circuits, such as masking
and etching or masking and ion milling, was used to pattern on the
substrate the groove 6, 0.1-0.2 .mu.m wide and 0.01 .mu.m or more
deep, along a center line of an area where the waveguide is to be
formed.
[0087] To prevent a possible, the film was formed by using
potassium oxide and KTa.sub.xNb.sub.1-xO.sub.3 oxide as a solvent
and a solute, respectively, and also using a self-fluxing method.
As the potassium oxide solvent, a mixture of KVO.sub.3 and K.sub.2O
(KVO.sub.3=30-70 mol %) or K.sub.2CO.sub.3 carbonate was used. When
a KTa.sub.xNb.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1) substrate was
used, a material KTa.sub.x'Nb.sub.1-x'O.sub.3 with x'
(0.ltoreq.x'.ltoreq.1) composition different from the substrate
composition was selected so that there was a difference in
refractive index between the substrate and the waveguide. Further,
0-10 mol % of LiCO.sub.3 was also added as an additive.
[0088] The above process resulted in a thin film with
KTa.sub.xNb.sub.1-xO.sub.3 replaced with K.sub.1-y
Li.sub.yTa.sub.xNb.sub.1-xO.sub.3 (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.0.5; x and y are compositions). The solute
concentration with respect to the solvent was set to 30-50 mol %, a
concentration range where the object material
KTa.sub.x'Nb.sub.1-x'O.sub.3 is shown in a phase diagram to
precipitate as an initial crystal. The film deposition temperature
was set at 0-10.degree. C. supercooled from a solid-solution
equilibrium temperature of 1,050-1,360.degree. C. determined from
the liquidus of the phase diagram and the concentration of the
solute used. This temperature was used considering the fact that a
low film deposition rate constitutes one of conditions for forming
a high quality film. However, there was also a case where a growth
rate with a larger supercooling had to be used when considering a
lattice matching between the KTa.sub.x'Nb.sub.1-x'O.sub.3 and the
substrate material used and a reactivity between the solution and
the substrate used. The film deposition rate achieved was around 1
.mu.m/min. The film deposition time was determined from the
precalculated deposition rate and the desired film thickness.
[0089] The crystal quality of the KTa.sub.x'Nb.sub.1-x'O.sub.3 film
deposited to a thickness of 2 .mu.m was best when the substrate was
formed from the same material or constitutional elements as the
film material, i.e., the material that satisfies the homo epitaxial
condition. The KTa.sub.x'Nb.sub.1-x'O.sub.3 film thus formed has an
optical quality of 0.10 dB/cm in terms of optical transmission loss
at a wavelength of 1.55 .mu.m. The KTa.sub.x'Nb.sub.1-x'O.sub.3
film was verified by the X-ray diffraction method to be oriented in
a <100> axis direction.
[0090] As for the structure of the waveguide obtained, it extends
along the groove and has a rectangular cross section 2 .mu.m wide
with the groove in the substrate as a center. The observation using
a SEM (scanning electron microscope) showed that the sidewall
surfaces of the waveguide had microsteps made up of {100} surfaces
with a better planarity than that of the sidewall surfaces of the
waveguide fabricated by the conventional photolithography. Strictly
speaking, its cross section had projections in the groove region at
positions of crystal growth nuclei but since the projections were
small compared with the rectangular cross-sectional area, their
influence on the transmitted light was small.
[0091] Rather than providing the groove structure on the substrate,
forming a groove or holes where crystal growth nuclei were to be
created was able to produce the similar effect.
[0092] FIG. 3 shows a substrate formed with a groove at positions
of crystal growth nuclei for the KTa.sub.xNb.sub.1-xO.sub.3
crystal. In the figure, reference numeral 11 denotes the substrate
and 16 the groove.
[0093] FIG. 4 shows a substrate formed with holes at positions of
crystal growth nuclei for the KTa.sub.xNb.sub.1-xO.sub.3 crystal.
In the figure, reference numeral 11 denotes the substrate and 17
the holes.
[0094] Instead of processing the substrate, forming a groove or
holes in the electrode layer on the substrate was able to produce
the similar effect.
[0095] FIG. 5 shows a substrate having an electrode layer formed
with holes at positions of crystal growth nuclei for the
KTa.sub.xNb.sub.1-xO.sub.3 crystal. FIG. 6 is a cross section taken
along the line VI-VI of FIG. 5. In the figure, reference number 11
denotes the substrate and 18 the holed electrode layer.
Embodiment 3
Embodiment 3-1
[0096] Next, a method of manufacturing a diffused waveguide by
diffusing Li ions in a crystal of the core formed from a crystal
material of KTa.sub.xNb.sub.1-xO.sub.3 composition will be
explained.
[0097] A KTN crystal plate 10 mm square and 0.5 mm thick which was
optically polished on both sides was put in a platinum boat
together with LiNO.sub.3 powder. They were heated in the atmosphere
at 300.degree. C., 400.degree. C., 500.degree. C. and 550.degree.
C. for 10 hours each. LiNO.sub.3 with a melting point of
261.degree. C. melted when heated and the KTN crystal plate was
soaked in the LiNO.sub.3 liquid. Then, the specimen was cooled in a
furnace down to a room temperature and then the boat was taken out.
In either case, LiNO.sub.3 was in a solid form when taken out.
Hence, the boat was washed with pure water and the crystal
substrate was taken out. The diffusion state of Li ions in the
specimen was analyzed by the SIMS (secondary ion mass
spectrometry).
[0098] Of the specimens described above, the one heat-treated at
550.degree. C. appeared opaque white at the surface and partly
flaked. This is considered due to the phenomenon in which the Li
ion concentration at the crystal surface becomes locally too high
causing a density difference with respect to the KTN crystal.
Hence, it is desired that the ion diffusion be performed at or
below 500.degree. C.
[0099] In this embodiment, a Li ion diffusion profile obtained from
the 500.degree. C. heat treatment is shown in FIG. 7. As can be
seen from FIG. 7, it is verified that the heat treatment in
LiNO.sub.3 has resulted in the ion diffusion. A temperature
increase caused an increase in the ion concentration and the
diffusion distance. The crystal diffused with Li was heated in the
atmosphere at 700.degree. C. for 10 hours to perform an internal
diffusion of Li ions.
[0100] A SIMS analysis result for Li ions is shown in FIG. 8. FIG.
9 shows a refractive index distribution estimated by the IWKB
method using an effective refractive index measured in each mode by
a prism coupling. As can be seen from FIG. 8 and FIG. 9, a
moderately curved refractive index distribution is formed. A
relative index difference on the surface portion is 1% or higher
and it is apparent that the Li diffusion can implement an index
difference large enough to function as a waveguide.
[0101] Further, while in this embodiment the internal diffusion was
performed in the atmosphere, it may be done in the presence of
oxygen, inert gas, water vapor or a mixture of these gases to
produce the similar effects.
[0102] Using the photolithography, a Pt mask pattern was formed on
the KTN crystal surface and a linear portion 1 .mu.m wide not
deposited with Pt was formed. The crystal was subjected to the Li
diffusion in the same way as in Embodiment 3-1. The heat treatment
in LiNO.sub.3 was performed at 400.degree. C. for 10 hours, after
which the internal diffusion was carried out in the presence of
oxygen at 700.degree. C. for 5 hours. Then, the Pt mask was etched
away by nitric acid. The crystal used in this embodiment is 20 mm
square and 0.5 mm thick.
[0103] Therefore, this method can form a linear waveguide 20 mm
long. The waveguide thus formed was a single-mode waveguide with a
mode field diameter of 8 .mu.m at a wavelength of 1.55 .mu.m. After
being applied with anti-reflection coating at both end faces, the
waveguide was aligned and connected with single-mode fibers using
UV resin. The insertion loss measured at the wavelength of 1.55
.mu.m was 2.5 dB and the waveguide loss taking the losses at end
faces into account was 0.1 dB/cm.
[0104] As described above, with the method of Embodiment 3-1 of
this invention, it is seen that a single-mode waveguide with small
losses can be fabricated. By changing the diffusion temperature in
the range of between 300.degree. C. and 500.degree. C. and the
diffusion time in the range of between 2 and 100 hours, it was
found possible to change the mode field diameter arbitrarily.
Embodiment 3-2
[0105] The Li diffusion was performed under the same condition as
in Embodiment 3-1 and Pt was etched, after which the crystal was
heated in a liquid of melted KNO.sub.3 at 400.degree. C. for five
hours. When the crystal containing Li ions is heated in the liquid
of melted KNO.sub.3, Li ions diffuse into the KNO.sub.3 liquid from
those portions of the crystal surface where the Li concentrations
are high, and K ions in the KNO.sub.3 liquid diffuse into the
crystal. As a result, near the surface of the crystal, Li ions are
replaced with K ions, reducing the Li ion concentrations.
[0106] Then, the specimen was cooled down to a room temperature and
the solidified KNO.sub.3 was washed away to recover the crystal
substrate. The crystal substrate was then heated in the atmosphere
at 650.degree. C. for 10 hours to obtain a crystal formed with a
waveguide.
[0107] The Li ion distribution in the waveguide manufactured
according to this method was measured by the SIMS in the depth
direction and in the horizontal direction. The result of
measurement is shown in FIG. 10. As shown in FIG. 10, in addition
to the method of Embodiment 3-1 described above, performing the
heat treatment in KNO.sub.3 made it possible to fabricate a
waveguide with an almost symmetrical ion distribution. This
waveguide exhibited no difference in light transmission
characteristic between a TE mode and a TM mode. With the method of
this embodiment, a diffused waveguide was obtained which has no
polarization dependency. Its waveguide loss as measured by a
technique similar to that used in Embodiment 3-1 was 0.13 dB/cm and
even the heat treatment in KNO.sub.3 resulted in no significant
increase in loss. It is therefore found that the method of this
embodiment is effective in making the waveguide independent of
polarization. Reference number 21 denotes the surface, 22 the Li
ion diffusion area (core), and 23 the cladding.
Embodiment 3-3
[0108] On the linear waveguide fabricated by the method of
Embodiment 3-1 an electrode pattern with a period of about 12 .mu.m
was formed. The opposite surface was deposited with gold to form a
lower electrode. The crystal was placed on a Peltier element so
that its temperature can be controlled. The cutoff wavelength for
the multimode is 0.7 .mu.m and, for longer wavelengths, the
waveguide functions as a single-mode waveguide. The length of the
waveguide manufactured was 3 cm and the loss of the waveguide was
0.15 dB/cm.
[0109] FIG. 11 shows the fabrication of a device manufactured in
this embodiment. An electrode pitch corresponds to a grating pitch
that realizes a quasi-phase matching required to perform a
wavelength conversion on a 1.55 .mu.m band with a 0.775 .mu.m light
used as a pump light. In this case, the electrode pitch is 12
.mu.m. A voltage corresponding to 1 kV/cm was applied to the
electrode; using polarization maintaining fibers, a signal light of
1.54 .mu.m and a pump light of 0.775 .mu.m were simultaneously
launched into the device; and an output light was measured using an
optical spectrum analyzer. Reference number 34 designates a Li
diffused waveguide, 35 an upper electrode, 36 a lower electrode,
and 37 a Peltier element.
[0110] FIG. 12 shows a spectrum of light after it was
wavelength-converted, with [a] representing a signal light, [b] a
second-order diffracted light of the pump light and [c] a converted
light. As can be seen from FIG. 12, the conversion efficiency was
calculated to be about 20 times that of the quasi-phase matching
device of LiNbO.sub.3. In this wavelength conversion, the light is
confined in the diffused core and the nonlinearity coefficient of
the core determines the efficiency of the wavelength conversion
device. As shown in this embodiment, it is found that the
nonlinearity of the KTN waveguide with diffused Li is very high and
is not degraded before or after the Li diffusion.
Embodiment 4
[0111] The present invention is characterized in that diketone
complex as a starting material of K is used to fabricate a crystal
film of KTa.sub.1-xNb.sub.xO.sub.3 (0<x<1) (hereinafter
referred to simply as KTN) and that metal .beta.-diketone
complexes-as starting material of K and Li are used to fabricate
crystal films of K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3
(0<x<1 and 0<y<1) (referred to simply as KLTN). The
.beta.-diketone complex of K and .beta.-diketone complex of Li are
defined in general expressions (1) and (2), respectively: ##STR2##
(In the above expressions, R is an alkyl group with a carbon number
of between 1 and 7; R' is an alkyl group or C.sub.nF.sub.2+1; and
n=1 to 3)
[0112] As can be seen from the general expressions (1) and (2), the
metal .beta.-diketone complexes have a structure in which an
organic substance coordinates with metal ions through oxygen. This
structure is stabilized by electrons becoming unlocalized in a
six-membered ring consisting of oxygen and carbon including K or
Li.
[0113] The electron state of the six-membered ring can be
controlled by the two kinds of substituents R and R' bonding to
carbon elements. These substituents determine a three-dimensional
structure, so if they are structured to enclose K or Li, the
volatility can be increased. The use of the metal .beta.-diketone
complexes can vaporize K or Li at temperatures below 300.degree. C.
to secure a vapor pressure necessary for the CVD method.
[0114] In this compound, however, the volatilization and
decomposition generally conflict with each other, so it is
important to properly select the substituents R and R' to improve
the stability for decomposition. To secure the stability against
decomposition it is desired that the substituents be alkyl groups
with a large carbon number. On the other hand, to increase the
volatility it is effective to replace hydrogen of the alkyl group
with fluorine. In this invention, therefore, when an emphasis is
placed on the thermal stability against decomposition, an alkyl
group or t-butyl group with a long normal chain is used. When the
volatility is emphasized, C.sub.nF.sub.2n+1 is used. In striking a
balance between stability and volatility, it is effective to use
the alkyl group as one substituent R and the fluorine-replaced
alkyl-group as the other R'.
[0115] More specifically, preferred .beta.-diketone complexes of K
include, but not limited to, those complexes having as ligands
2,2-dimethyl-3,5-octanedione,
2,2-dimethyl-6,6,6-trifuoro-3,5-hexanedione,
5,5,5-trifluoro-2,4-pentanedione,
2,2-dimethyl-6,6,7,7,7-pentafluoro-3,5-heptanedione, and
2,2-dimethyl-6,6,7,7,8,8,8-heptanefluoro-3,5-octanedione. Preferred
.beta.-diketone complexes of Li include, but not limited to, those
complexes with 2,6,6-tetramethyl-3,5-heptanedione as ligands.
[0116] The method of manufacturing a KTN crystal film of this
invention introduces, as a gas flow into a reaction system, the
.beta.-diketone complex of K described above, a gaseous compound
and/or a volatile compound of Ta, which is one of metal elements
making up the crystal film, a gaseous compound and/or a volatile
compound of Nb, and an oxygen-containing gas that works as an
oxidizing agent. In this reaction system a substrate is provided
and the introduced components are reacted in gas phase or on this
substrate to form a KTN crystal film on the substrate.
[0117] The method of manufacturing a KLTN crystal film of this
invention introduces, as a, gas flow into a reaction system, the
.beta.-diketone complexes of K and Li described above, a gaseous
compound and/or a volatile compound of Ta, which is one of metal
elements making up the crystal film, a gaseous compound and/or a
volatile compound of Nb, and an oxygen-containing gas that works as
an oxidizing agent. This reaction system is provided with a
substrate, and the introduced components are reacted in gas phase
or on this substrate to form a KLTN crystal film on the
substrate.
[0118] Among preferred gaseous and/or volatile compounds of Ta used
in this invention are alkoxide such as Ta(OC.sub.2H.sub.5).sub.5
and halide TaCl.sub.5. Preferred gaseous and/or volatile compounds
of Nb include alkoxide such as Nb(OC.sub.2H.sub.5).sub.5 and halide
NbCl.sub.5.
[0119] The oxygen-containing gas is preferably a gas containing
oxygen, or a gas containing oxygen and at least one of hydrogen and
nitrogen. When fluorinated alkyl is used as a substituent of
.beta.-diketone complex of K and/or .beta.-diketone complex of Li,
it is preferred that an oxygen-containing gas containing hydrogen
be used as a fluorine getter.
[0120] Among possible substrates for use in this invention are
SrTiO.sub.3, SiO.sub.2, MgO, MgAl.sub.2O.sub.4, NdGaO.sub.3,
KTa.sub.1-xNb.sub.xO.sub.3,
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3, and
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3. The temperature of the
substrate during reaction is preferably in the range of
400-1,200.degree. C. because the KTN or KLTN crystal structure
produced in this temperature range is homogeneous.
[0121] Further, since the .beta.-diketone complex of K and
.beta.-diketone complex of Li have their metal ions already bonded
with oxygen, they have an advantage of being able to produce an
oxide by thermal decomposition without using an oxidizer. It should
be noted, however, that an oxygen-containing gas is preferably
introduced for preventing contamination by carbon.
[0122] With this method, a fast film deposition rate of 100-150
.mu.m/hour is obtained. The crystal films of
KTa.sub.1-xNb.sub.xO.sub.3 and/or
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 fabricated in this
manner have a nonlinear optical effect and an electrooptical effect
and can be used in optical signal processing devices for wavelength
conversion, waveform shaping and optical amplification.
Embodiment 4-1
[0123] FIG. 13 is an outline of the CVD apparatus used in this
embodiment. The raw material of K used was
K(DPM)(2,2,6,6-tetramethyl-3,5-heptanedione complex) which has
t-butyl groups as both substituents. The Ta material and Nb
material used were Ta(OC.sub.2H.sub.5).sub.5 and
Nb(OC.sub.2H.sub.5).sub.5, respectively. K(DPM) is a solid material
while Ta(OC.sub.2H.sub.5).sub.5 and Nb(OC.sub.2H.sub.5 ).sub.5 are
liquid materials. These materials were introduced into
independently temperature-controlled bubblers, i.e., a Ta material
bubbler 41, a Nb material bubbler 42 and a K material bubbler 43,
and temperature-controlled by heaters (not shown).
[0124] As a material carrier gas, argon was used for K(DPM) and,
for other Ta and Nb materials, oxygen was used. The material
temperature was set at 200.degree. C. for K(DPM), 142.degree. C.
for Ta(CO.sub.2H.sub.5).sub.5 and 145.degree. C. for
Nb(OC.sub.2H.sub.5).sub.5. The amount of material supplied was
adjusted by the carrier gas flow. The piping temperature was
controlled at 205.degree. C. to prevent a possible condensation of
the material. After the material gases were mixed in a mixer 44,
they were introduced into a reaction tube 45. The pressure in the
apparatus was reduced to 1.3 kPa (10 Torr), and the substrate 47
was heated by an external heater (electric furnace) 46 to
600.degree. C., 700.degree. C., 800.degree. C., 900.degree. C.,
1,000.degree. C. and 1,100.degree. C. SrTiO.sub.3 was used for the
substrate. Reference numeral 41 denotes the Ta material bubbler, 42
the Nb material bubbler, 43 the K material bubbler, and 48 a rotary
pump.
[0125] The carrier gas was controlled so that the composition of
the crystal was KTa.sub.0.65Nb.sub.0.35O.sub.3. The films were
deposited to a thickness of 2.0 .mu.m. The crystal films formed at
different temperatures described above were observed by a SEM and
their crystal phases were identified by the X-ray diffraction. The
light transmission losses at the wavelength of 1.55 .mu.m were
measured by the prism coupling, and the homogeneity of each crystal
film was measured by measuring the strength distribution of a
second harmonic generation. The measurement results are shown in
Table 1. TABLE-US-00001 TABLE 1 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.08 Excellent *600 -- Polycrystalline 7 0.1 Excellent
700 0.5 Polycrystalline 10 0.1 Excellent 800 0.9 Polycrystalline 15
0.1 Excellent 900 1.5 Polycrystalline 25 0.1 Excellent 1,000 2.5
Single crystal -- 0.1 Excellent 1,100 5.0 Single crystal -- 0.1
Excellent *600 After the film was formed at 600.degree. C., it was
heat-treated for two hours at 1,000.degree. C. in the air.
[0126] The KTN film fabricated at a substrate temperature of
600.degree. C. was checked by the X-ray diffraction and found to be
amorphous. Its light transmission loss was 0.08 dB/cm, which is
sufficiently low for a waveguide film. The deposition rate was 0.3
.mu.m/min.
[0127] This film was heat-treated for two hours at 1,000.degree. C.
in the air for crystallization to produce a polycrystalline film.
The X-ray diffraction showed that after two hours of heat treatment
at 1,000.degree. C. in the atmosphere, the film was oriented in the
direction of the substrate SrTiO.sub.3. The SEM observation found
that the average grain diameter was 7 .mu.m. The light transmission
loss of this film was 0.1 dB/cm and there was no significant
increase in the scattering loss due to crystallization. Even with
an incident He--Ne laser beam, no light scattering was observed at
grain boundaries.
[0128] When the substrate temperature was set at 700.degree. C.,
800.degree. C. and 900.degree. C., polycrystalline films oriented
in the direction of the substrate were obtained. The deposition
rates at these temperatures-were 0.5 .mu.m/min. 0.9 .mu.m/min and
1.5 .mu.m/min, respectively. It is found that the grain diameter of
the crystal film increases with the temperature. At the temperature
of 900.degree. C. the average grain diameter reached 25 .mu.m. The
light transmission losses of these polycrystalline films were
around 0.1 dB/cm and the films were optically homogeneous.
[0129] For the substrate temperature of 1,000.degree. C., the SEM
observation and the X-ray diffraction analysis found that a single
crystal was obtained. The optical characteristic of this film was
almost identical with that of the polycrystalline films but with a
slightly lower scattering loss.
[0130] FIG. 14 is a graph showing a temperature-dependency of
dielectric constant of the crystal grown at the substrate
temperature of 1,000.degree. C. It is seen from this graph that the
dielectric constant at around the phase transition temperature
reaches as high as 30,000 and that this film is homogeneous and
exhibits nearly the same characteristics as those of the bulk
single crystals.
[0131] For the substrate temperature of 1,100.degree. C., a single
crystal film was able to be formed. But the volatilization of
K.sub.2O was remarkable and thus it was necessary to increase the
amount of supply of K(DPM).
Embodiment 4-2 to 4-7
[0132] Crystal films were formed at predetermined temperatures
(600, 700, 800, 900, 1,000 and 1,100.degree. C.) in a manner
similar to Embodiment 4-1, except that the following materials were
used for the substrate instead of SrTiO.sub.3 used in Embodiment
4-1: SiO.sub.2 (Embodiment 4-2), MgO (Embodiment 4-3),
MgAl.sub.2O.sub.4 (Embodiment 4-4), NdGaO.sub.3 (Embodiment 4-5),
KTa.sub.1-xNb.sub.xO.sub.3 and
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 (Embodiment 4-6), and
K.sub.1-yLi.sub.yTa.sub.1-xNb.sub.xO.sub.3 (Embodiment 4-7).
[0133] The crystal films thus formed were subjected to examinations
similar to those of Embodiment 4-1, i.e., identification of crystal
phase and measurement of crystal grain diameter, light transmission
loss and homogeneity. The results of examinations are similar to
those of Embodiment 4-1 and shown in Table 2 (Embodiment 4-2),
Table 3 (Embodiment 4-3), Table 4 (Embodiment 4-4), Table 5
(Embodiment 4-5), Table 6 (Embodiment 4-6), and Table 7 (Embodiment
4-7). TABLE-US-00002 TABLE 2 Light Deposi- Average trans- Substrate
tion grain mission tempera- rate dia. loss Homogene- ture (.degree.
C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3 Amorphous
-- 0.09 Excellent 700 0.5 Amorphous -- 0.08 Excellent 800 1.0
Amorphous -- 0.08 Excellent 900 1.5 Amorphous -- 0.09 Excellent
1,000 2.6 Polycrystalline 5 0.12 Excellent 1,100 5.1
Polycrystalline 10 0.11 Good
[0134] TABLE-US-00003 TABLE 3 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.09 Excellent 700 0.5 Polycrystalline 5 0.12
Excellent 800 1.0 Polycrystalline 7 0.13 Excellent 900 1.5
Polycrystalline 10 0.12 Excellent 1,000 2.6 Single crystal -- 0.12
Excellent 1,100 5.0 Single crystal -- 0.11 Good
[0135] TABLE-US-00004 TABLE 4 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.09 Excellent 700 0.5 Polycrystalline 5 0.10
Excellent 800 1.0 Polycrystalline 7 0.11 Excellent 900 1.5 Single
crystal -- 0.10 Excellent 1,000 2.5 Single crystal -- 0.10
Excellent 1,100 5.0 Single crystal -- 0.09 Excellent
[0136] TABLE-US-00005 TABLE 5 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.08 Excellent 700 0.5 Polycrystalline 6 0.11
Excellent 800 1.1 Polycrystalline 8 0.11 Excellent 900 1.5
Polycrystalline 15 0.10 Excellent 1,000 2.6 Single crystal -- 0.10
Excellent 1,100 5.1 Single crystal -- 0.09 Excellent
[0137] TABLE-US-00006 TABLE 6 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.08 Excellent 700 0.6 Polycrystalline 7 0.11
Excellent 800 1.1 Polycrystalline 10 0.10 Excellent 900 1.5 Single
crystal -- 0.10 Excellent 1,000 2.6 Single crystal -- 0.10
Excellent 1,100 5.2 Single crystal -- 0.09 Excellent
[0138] TABLE-US-00007 TABLE 7 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3
Amorphous -- 0.08 Excellent 700 0.5 Polycrystalline 6 0.10
Excellent 800 1.1 Polycrystalline 9 0.10 Excellent 900 1.5 Single
crystal -- 0.10 Excellent 1,000 2.6 Single crystal -- 0.10
Excellent 1,100 5.0 Single crystal -- 0.10 Excellent
[Embodiment 4-8 to 4-12
[0139] Crystal films were formed at predetermined temperatures
(600, 700, 800, 900, 1,000 and 1,100.degree. C.) in a manner
similar to Embodiment 4-1, except that the ligands of the
.beta.-diketone complex as the K material were
2,2-dimethyl-3,5-octanedione (Embodiment 4-8),
2,2-dimethyl-6,6,6-trifluoro-3,5-hexandione (Embodiment 4-9),
5,5,5-trifluoro-2,4-pentanedione (Embodiment 4-10),
2,2-dimethyl-6,6,7,7,7-pentafluoro-3,5-heptanedione (Embodiment
4-11), 2,2-dimethyl-6,6,7,7,8,8,8-heptanefluoro-3,5-octanedione
(Embodiment 4-12).
[0140] The crystal films thus formed were subjected to examinations
similar to those of Embodiment 4-1, i.e., identification of crystal
phase and measurement of crystal grain diameter, light transmission
loss and homogeneity. In these Embodiments 4-8 to 4-12, the crystal
films obtained have characteristics similar to those of Embodiment
4-1. When a material containing fluorine was used (Embodiments 4-9,
4-10, 4-11, 4-12), there was a decrease in the deposition rate.
However, introducing H.sub.2O into the reaction system prevented
the deposition rate reduction.
[0141] The results of measurements are shown in Table 8 (Embodiment
4-8), Table 9 (Embodiment 4-9), Table 10 (Embodiment 4-10), Table
11 (Embodiment 4-11), and Table 12 (Embodiment 4-12).
TABLE-US-00008 TABLE 8 Light Deposi- Average trans- Substrate tion
grain mission tempera- rate dia. loss Homogene- ture (.degree. C.)
(.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.3 Amorphous --
0.08 Excellent 700 0.5 Polycrystalline 10 0.10 Excellent 800 1.0
Polycrystalline 15 0.10 Excellent 900 1.5 Polycrystalline -- 0.10
Excellent 1,000 2.5 Single crystal -- 0.10 Excellent 1,100 5.0
Single crystal -- 0.09 Excellent
[0142] TABLE-US-00009 TABLE 9 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600
0.15 Amorphous -- 0.08 Excellent 700 0.25 Polycrystalline 12 0.10
Excellent 800 0.5 Polycrystalline 17 0.10 Excellent 900 0.7
Polycrystalline -- 0.10 Excellent 1,000 1.2 Single crystal -- 0.10
Excellent 1,100 2.3 Single crystal -- 0.09 Excellent
[0143] TABLE-US-00010 TABLE 10 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600
0.12 Amorphous -- 0.08 Excellent 700 0.25 Polycrystalline 13 0.10
Excellent 800 0.4 Polycrystalline 17 0.10 Excellent 900 0.6
Polycrystalline -- 0.10 Excellent 1,000 1.2 Single crystal -- 0.10
Excellent 1,100 2.2 Single crystal -- 0.09 Excellent
[0144] TABLE-US-00011 TABLE 11 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600
0.11 Amorphous -- 0.08 Excellent 700 0.21 Polycrystalline 13 0.10
Excellent 800 0.39 Polycrystalline 19 0.10 Excellent 900 0.8
Polycrystalline -- 0.10 Excellent 1,000 1.2 Single crystal -- 0.10
Excellent 1,100 2.3 Single crystal -- 0.09 Excellent
[0145] TABLE-US-00012 TABLE 12 Light Deposi- Average trans-
Substrate tion grain mission tempera- rate dia. loss Homogene- ture
(.degree. C.) (.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600
0.11 Amorphous -- 0.08 Excellent 700 0.2 Polycrystalline 12 0.10
Excellent 800 0.35 Polycrystalline 19 0.10 Excellent 900 0.8
Polycrystalline -- 0.10 Excellent 1,000 1.3 Single crystal -- 0.10
Excellent 1,100 2.5 Single crystal -- 0.09 Excellent
Embodiment 4-13
[0146] Crystal films were fabricated at predetermined temperatures
(600, 700, 800, 900, 1,000 and 1,100.degree. C.) in a manner
similar to Embodiment 4-1, except that Li(DPM) was used in addition
to K(DPM) as the initial material and that TaCl.sub.5 and
NbCl.sub.5 were also used. The compositions of these films thus
formed were K.sub.0.9Li.sub.0.1Ta0.65Nb.sub.0.35O.sub.3. These
crystal films were subjected to examinations similar to those of
Embodiment 4-1, i.e., identification of crystal phase and
measurement of crystal grain diameter, light transmission loss and
homogeneity. The results of measurements are shown in Table 13.
TABLE-US-00013 TABLE 13 Light Deposi- Average trans- Substrate tion
grain mission tempera- rate dia. loss Homogene- ture (.degree. C.)
(.mu.m/min) Crystal state (.mu.m) (dB/cm) ity 600 0.28 Amorphous --
0.08 Excellent 700 0.48 Polycrystalline 12 0.10 Excellent 800 0.85
Polycrystalline 19 0.10 Excellent 900 1.2 Polycrystalline -- 0.10
Excellent 1,000 2.2 Single crystal -- 0.10 Excellent 1,100 3.8
Single crystal -- 0.09 Excellent
[0147] The measurement result indicates that the crystal films have
crystal states and transparent characteristics almost similar to
those of Embodiment 4-1 and that this method can produce a high
quality KLTN crystal.
[0148] FIG. 15 shows a temperature.-dependency of dielectric
constant of the crystal film formed at 1,000.degree. C. From the
measurement of dielectric constant it is evident that a homogeneous
film was produced.
Embodiment 4-14
[0149] FIG. 16A and FIG. 16B illustrate a fabrication of a
wavelength conversion device manufactured in this embodiment. FIG.
16A is a perspective view of the wavelength conversion device and
FIG. 16B is a cross-sectional view taken along the line XVIB-XVIB
of FIG. 16A.
[0150] A SiO.sub.2 substrate 54 with a lower electrode 50 of Au
formed by evaporation was heated to 900.degree. C. and deposited
with a KTN crystal film to a thickness of 5 .mu.m in a manner
similar to that of Embodiment 4-1. On the KTN crystal film a KLTN
crystal film was formed to a thickness of 0.4 .mu.m by the method
of embodiment 4-2. Both of the crystal films were polycrystalline
with a grain diameter of 15 .mu.m. The KLTN crystal film was
processed by photolithography into a ridge waveguide 52 0.4 .mu.m
wide. Then, a KTN crystal film was formed over the entire surface
of the substrate by the method of Embodiment 4-1. This KTN crystal
film was grown over the KLTN crystal film to a thickness of 3
.mu.m. In this way, a KLTN crystal film waveguide enclosed by the
KTN crystal film 51 was obtained.
[0151] The relative index difference of the manufactured waveguide
is 2.5% and the cutoff wavelength is 0.6 .mu.m. For longer
wavelengths the waveguide functions as a single-mode waveguide. The
length of the waveguide fabricated was 3 cm and the light
transmission loss of the waveguide 0.15 dB/cm.
[0152] An upper electrode 53 was formed over the KTN crystal film
by evaporating gold. An electrode pitch corresponds to a grating
pitch that realizes a quasi-phase matching required to perform a
wavelength conversion on a 1.55 .mu.m band with a 0.773 .mu.m light
used as a pump light. In this case, the electrode pitch is 12
.mu.m. Hence, this waveguide functions as a wavelength conversion
device.
[0153] In this way, a wavelength conversion device with electrodes
as shown in FIG. 16 was fabricated. A voltage corresponding to 1
kV/cm was applied to the electrode; using polarization maintaining
fibers, a signal light of 1.54 .mu.m and a pump light of 0.773
.mu.m were simultaneously launched into the device; and an output
light was measured using an optical spectrum analyzer.
[0154] FIG. 12 shows a spectrum of light after it was
wavelength-converted. In the figure, [a] represents the wavelength
of a signal light, [b] the wavelength of a second-order diffracted
light of the pump light and [c] the wavelength of a converted
light. FIG. 12 clearly indicates that the wavelength conversion is
realized by the differential frequency generation. Further, the
signal light and the converted light were parametric-amplified and
the gain of the converted light with respect to the input signal
light reaches as high as about 15 dB, which cannot be realized with
the conventional LN wavelength conversion devices. As can be seen
from this diagram, the method of this invention makes it possible
to arrange in layer functional KTN or KTLN waveguides on a
SiO.sub.2 substrate.
Embodiment 4-15
[0155] A wavelength conversion device was manufactured in a way
similar to that of Embodiment 4-14, except that the substrate
temperature was set at 1,000.degree. C. Although the crystal formed
was a single crystal as the substrate temperature was changed,
characteristics obtained were similar to those of Embodiment
4-14.
[0156] The present invention has been described in detail with
respect to preferred embodiments, and it will now be apparent from
the foregoing to those skilled in the art that changes and
modifications may be made without departing from the invention in
its broader aspects, and it is the intention, therefore, in the
appended claims to cover all such changes and modifications as fall
within the true spirit of the invention.
* * * * *