Optical guided-wave device and manufacturing method

An optical guided-wave device with an electro optic effect is comprised of first and second substrates having first and second refractive indices wherein the second refractive index is larger than the first one. These substrates are made of a single crystal dielectric material such as lithium tantalate or lithium niobate and, if they are made of the same material, they have different crystal orientations resulting in different refractive indices. These substrates are physically bonded directly or via a thin film such as glass, silicon, silicon oxide or silicon nitride and then, the second substrate is thinned and worked to form a wave guide therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a structure for improving the performance 
of various optical guided-wave devices using an optical wave guide for 
such applications as optical power modulation, optical switching, plane of 
polarization control, optical phase matching and propagation mode control, 
and further relates to the manufacturing method of said structure. 2. 
Description of the Prior Art 
Conventional optical guided-wave devices such as optical modulators, 
optical switches, plane of polarization control devices, optical phase 
matching and optical propagation mode control devices form a single 
propagation mode optical wave guide in a dielectric single crystal having 
an electro-optic effect (such as lithium niobate (LiNbO.sub.3) or lithium 
tantalate (LiTaO.sub.3)), and control the passage of light through the 
optical wave guide by manipulating the shape of the optical wave guide, 
providing appropriately shaped electrodes, and utilizing the electrooptic 
effect. The structure of such optical guided-wave devices is described in 
Waveguide Electrooptic Modulators by R. Alferness, (IEEE Transactions on 
Microwave and Techniques, Vol. MTT-30, No. 8, 1121-1137 (August, 1989)). 
Manufacturing methods for optical wave guides are likewise described in 
Optical Waveguide Modulators by I. Kaminow (IEEE Transactions on Microwave 
and Techniques, Vol. MTT-23, No. 1, 57-70 (1975)). 
In one such manufacturing method, lithium niobate or lithium tantalate is 
heat treated at a high temperature to modify the refractive index of the 
material by out-diffusing the lithium. Alternatively, a metallic film of, 
for example, titanium is formed by vapor deposition and thermally diffused 
at a high temperature to raise the refractive index of the diffused area 
slightly above that of the surrounding area. In either case, the 
difference in refractive indexes is used to trap light. 
An example of a Mach-Zehnder type optical modulator using a titanium 
diffusion is described in Japanese patent laid-open publication SHO 
63-261219. In another method described in the literature, a metallic mask 
is formed over the specified areas and a proton-ion exchange is induced in 
phosphoric acid at 200.degree. C. to 300.degree. C., partially modifying 
the refractive index and forming the optical wave guide. Manufacturing 
methods that rely on out-diffusion, thermal diffusion, or ion exchange 
from the surface all form the optical wave guide by means of diffusion 
from the surface. The cross section of the optical wave guide is therefore 
necessarily determined by the diffusion process, resulting in numerous 
problems. 
One of the biggest problems is coupling loss between the optical wave guide 
and the optical fiber. While the cross section of an optical fiber is 
circular, the shape of most conventional optical wave guides is roughly an 
inverted triangle due to formation of the optical wave guide by diffusion 
from the surface. Because the strength of the guided light is greatest 
near the surface, optical coupling with the optical fiber is poor, 
resulting in significant loss. Reducing this optical coupling loss is 
therefore an extremely important topic in optical guided-wave device 
design. 
Another problem caused by diffusion processing is greater optical 
propagation loss after diffusion processing than before. With a titanium 
diffusion optical wave guide, for example, propagation loss of 
approximately 1 dB/cm normally occurs. Reducing propagation loss is 
therefore another major topic in optical guided-wave device design. 
A third problem is the increase in optical damage resulting from diffusion 
processing. Optical damage refers to the increase in propagation loss over 
time when a high intensity light source or a short wavelength light source 
is input to a diffusion-type optical wave guide. This is believed to be 
caused by the diffusion of ions in the optical wave guide resulting in 
increased trapping of electrons in the optical wave guide. 
It should be noted that methods for forming an optical wave guide without 
relying on diffusion processing have been described. One of these is 
described by Kaminow (see above reference). In this method, lithium 
niobate crystals are grown on top of a lithium tantalate layer, or a 
lithium niobate thin-film is formed by sputtering on top of a lithium 
niobate or lithium tantalate layer, and the optical wave guide is formed 
in this lithium niobate top layer. A similar method is described in 
Japanese patent laid-open publication SHO 52-23355. This method also forms 
an epitaxial growth lithium niobate top layer over a substrate of lithium 
tantalate (e.g.) using liquid phase, gas phase, fusion, or other method, 
and forms the optical wave guide in this top layer. There are, however, 
several problems with these optical wave guide formation methods using 
such thin-film crystal growth technologies. First, it is extremely 
difficult to achieve a thickness of greater than 5 .mu.m in epitaxial 
growth films, and productivity is accordingly poor, because of the growth 
speed and flaws occurring in the crystals while being grown. In addition, 
the coupling characteristics of a thin-film less than 5 .mu.m thick with 
an optical fiber having a core diameter of approximately 10 .mu.m are also 
poor. (The fiber core being where the light is trapped.) 
Productivity is further hampered because a good quality single crystal 
thin-film cannot be obtained unless the lattice constants of the thin film 
is essentially same as those of the substrate. It is therefore extremely 
difficult to form a good lithium niobate thin-film on a lithium tantalate 
substrate, and a mixed niobium-tantalum crystal film is often used. Pure 
lithium niobate, however, offers superior overall optical wave guide 
characteristics when compared with a mixed crystal film. 
While epitaxial growth of like materials is possible, the crystal 
orientation of the two layers will be the same, making it difficult to 
obtain an effective difference between the refractive index of the base 
substrate and that of the grown thin-film. This results in a solid 
substrate in which the optical wave guide cannot be formed. 
If the thin-film formed by these thin-film growth technologies is not good, 
propagation loss will increase and optical damage will increase even when 
the layers are stacked thickly, and the resulting film is therefore not 
desirable. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a manufacturing 
method and structure for an optical guided-wave device characterized by 
minimal optical fiber coupling loss, minimal propagation loss, and minimal 
optical damage. 
To achieve this object, an optical guided-wave device according to the 
present invention has an electro-optic effect and comprises an optical 
wave guide formed in a wafer of at least two bonded single crystal 
dielectric body substrates. The refractive indexes of these substrates 
differ and are determined by the crystal orientation of the dielectric 
body when the substrates are of like materials, or are determined by the 
basic material composition when the substrates are of different materials. 
The substrates are directly bonded, or are bonded through a glass film, 
silicon film, silicon oxide film, or silicon nitride film formed at a 
predetermined place on the substrates. Guided light is trapped inside one 
of the single crystal dielectric substrates due to the difference in the 
refractive indexes of the substrates. The optical guided-wave device 
controls the light passing through the optical wave guide by means of the 
electro-optic effect of the device.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiments of an optical guided-wave device according to the 
present invention as applied to an optical modulator, and the 
manufacturing methods of such devices, are described below with reference 
to the accompanying figures. 
First Embodiment 
The structure of the first embodiment is shown in FIGS. 1 and 2. FIG. 1 is 
an oblique view of the optical guided-wave device as applied in an optical 
modulator comprising a lithium niobate substrate 1, a lithium niobate 
thin-plate 2 bonded onto the lithium niobate substrate 1, an input/output 
(I/O) optical wave guide 3 formed on the lithium niobate thin-plate 2, 
first and second optical wave guide branches 4 and 5, and electrodes 6, 7 
formed on both sides of the second optical wave guide branch 5. 
The lithium niobate thin-plate 2 is directly bonded to the lithium niobate 
substrate 1 by cleaning and hydrophilic processing of the surface of each 
substrate before direct heat bonding. The crystal orientation of the 
lithium niobate thin-plate 2 is different from that of the lithium niobate 
substrate 1. 
The optical wave guide branches 4 and 5 are formed by splitting the input 
side of the optical wave guide 3. 
FIG. 2 is a cross section of FIG. 1 at a section through the first and 
second optical wave guide branches 4 and 5. Like reference numerals 
indicate the same components in FIGS. 1 and 2. The optical wave guide 
branches 4 and 5 and the I/O optical wave guide 3 have a trapezoidal cross 
section characteristic of a so-called ridge-type optical wave guide. The 
guided light propagators 8 are located below the optical wave guide 
branches 4 and 5. 
The construction of this optical modulator is known as a Mach-Zehnder 
design. Incident light entering the input branches into two parts. An 
electrical field is applied to one of the optical wave guide branches, 
creating an electro-optic effect that modifies the refractive index of the 
optical wave guide, changes the propagation speed of the guided light, and 
thus offsets the phase of the light when the branches rejoin. The effect 
of this is to modulate the strength of the light at the-output. 
The dielectric constant of lithium niobate parallel to the optical axis of 
the crystal is significantly different from the dielectric constant 
perpendicular to the optical axis, resulting in a corresponding difference 
in the refractive index in each direction. The refractive index to 
ordinary light is approximately 2.29 while the refractive index to 
extraordinary light is 2.20. If there is a difference of greater than 
approximately 0.01 in the refractive indexes, light can be trapped in the 
area with the higher refractive index, thus making it possible to form an 
optical wave guide. 
In this embodiment the crystal axes of the lithium niobate substrate 1 and 
the lithium niobate thin-plate 2 are selected so that the refractive index 
of the lithium niobate thin-plate 2 is greater than that of the lithium 
niobate substrate 1 for the optical propagation mode. As a result, light 
incident to the lithium niobate thin-plate 2 is trapped inside the 
thin-plate. By additionally employing a ridge construction, the effective 
dielectric constant of the area under the ridges is greater than that of 
the other (thinner) areas, thus trapping light below the ridges and 
enabling the under-ridge area to act as an optical wave guide. 
The shape of the optical wave guide in this case is either trapezoidal or 
rectangular in end cross section with a uniform refractive index, thus 
placing the center of the guided light near the center of the optical wave 
guide with an approximately circular cross section. The cross section of 
the I/O optical wave guide 3 is the same. This results in extremely good 
coupling efficiency between the round I/O optical wave guide structure and 
the core (approx. 10 .mu.m diameter) of the optical fiber. 
Typical values for these dimensions are a lithium niobate substrate 1 
thickness of 600 .mu.m, lithium niobate thin-plate 2 optical wave guide 
thickness of 7 .mu.m, ridge height at the peak of 3 .mu.m, optical wave 
guide width of 7 .mu.m, optical wave guide branch length of 2 cm, and 
total optical wave guide length of 3 cm. The electrodes are aluminum. 
With this construction, optical fiber coupling loss is less than 0.3 dB at 
one plane when coupled with adhesive bonding using an adhesive with an 
adjusted refractive index. This is a significant improvement over the 
typical coupling loss of 0.5-1.0 dB of an optical guided-wave device using 
a conventional titanium diffusion optical wave guide and the same adhesive 
bonding method. The performance of the optical modulator itself is 
equivalent to that of the conventional titanium diffusion optical wave 
guide. 
Optical propagation loss of the optical wave guide is also significantly 
reduced because a lithium niobate thin-plate with the optical 
characteristics of pure single crystals is used without ion diffusion 
processing. Specifically, optical wave guide propagation loss of less than 
0.1 dB/cm is easily obtained. This is also a significant improvement over 
the conventional titanium diffusion optical wave guide in which 
propagation loss ranges from 0.5-1.0 dB/cm. 
The strength of the input light was also varied from 0 dBm to 30 dBm to 
determine the optical damage characteristic, but virtually no optical 
damage was observed. This is attributed to the use of a pure single 
crystal lithium niobate thin-plate with an extremely low level of trapped 
electrons as the optical wave guide. 
Measurements were made using a 1.3 .mu.m wavelength light. 
Second Embodiment 
The structure of the second embodiment is shown in FIGS. 3 and 4. FIG. 3 is 
an oblique view of the optical guided-wave device as applied in an optical 
modulator, and FIG. 4 is a cross section of FIG. 3 at a section through 
the first and second optical wave guide branches 4 and 5. This embodiment 
differs from the first in that a lithium niobate thin-plate 12 is directly 
bonded to a lithium tantalate substrate 11 by cleaning and hydrophilic 
processing of the surface of both substrates before direct heat bonding. 
Identical parts in the first and second embodiments are identified by the 
same reference numerals. 
The refractive indices of lithium tantalate and lithium niobate differ. The 
refractive index to ordinary light of lithium tantalate is 2.175 while 
that of lithium niobate is 2.29. This results in an appropriate difference 
of 0.115. Light can therefore be trapped in the lithium niobate layer with 
the higher refractive index, thus forming an optical wave guide. Light 
incident to the lithium niobate thin-plate 12 is therefore trapped inside 
the thin-plate. By additionally employing a ridge construction, the 
effective dielectric constant of the area under the ridges is greater than 
that of the other (thinner) areas, thus trapping light below the ridges 
and enabling the under-ridge area to act as an optical wave guide. 
The shape of the optical wave guide in this embodiment is identical to that 
of the first embodiment, and coupling efficiency with the round optical 
wave guide structure of the optical fiber is extremely good. 
Typical values for these dimensions are a lithium tantalate substrate 11 
thickness of 600 .mu.m with all other values the same as in the first 
embodiment above. 
With this construction, optical fiber coupling loss is less than 0.3 dB as 
in the first embodiment, a significant improvement over the conventional 
model. 
Optical wave guide propagation loss of less than 0.1 dB/cm is easily 
obtained, again as in the first embodiment. The results of optical damage 
observations were also the same as in the first embodiment above. 
Third Embodiment 
This third embodiment describes a first embodiment of a manufacturing 
method for an optical guided-wave device according to the invention. 
First, the surfaces of two lithium niobate wafers with different crystal 
orientations and each mirror polished are cleaned with an etching process. 
Specifically, the surface layer of the lithium niobate wafer is etched 
away using a hydrofluoric acid etching agent. The surfaces are then 
flushed in demineralized water and immediately sandwiched uniformly 
together, easily enabling direct bonding by the water, hydroxyl groups, 
and hydrogen adsorbed in the lithium niobate wafer surfaces. While this 
process yields a sufficiently strong bond, the bond is further 
strengthened by heat treatment at a temperature between 100.degree. C. and 
1100.degree. C. 
The lithium niobate wafer with the crystal orientation having the higher 
refractive index is then mechanically polished and etched to a thin-plate 
layer. After reducing the wafer to a thickness of 7 .mu.m, an etching mask 
is formed on the thinned lithium niobate wafer to the pattern of the 
optical wave guide structure shown in the first embodiment using 
photolithography techniques, and the unmasked areas (the area not forming 
the optical wave guide) are removed to a depth of 3 .mu.m by etching. The 
mask is Cr and the etching agent is a hydrofluoric acid etching solution. 
The mask is then removed, and the aluminum electrodes are formed using 
conventional photolithography and etching technologies. 
This process yields the structure of the optical guided-wave device as 
shown in the first embodiment. The coupling characteristic, propagation 
loss, and optical damage characteristic of this optical guided-wave device 
and optical fiber are as described above with reference to the first 
embodiment. 
Fourth Embodiment 
This fourth embodiment describes a second embodiment of a manufacturing 
method for an optical guided-wave device according to the invention. 
As in the third embodiment above, the surfaces of a lithium niobate wafer 
and a lithium tantalate wafer, which are ground to a mirror finish, are 
cleaned with an etching process. The subsequent process is the same as in 
the third embodiment above, resulting in direct bonding of the lithium 
tantalate and lithium niobate wafers. After reducing the thickness of the 
lithium niobate wafer, which has a higher refractive index, to 7 .mu.m, 
the wafer is masked, etched, and aluminum electrodes are formed as 
described in the third embodiment, resulting in an optical guided-wave 
device constructed as described in the second embodiment above. 
The coupling characteristic, propagation loss, and optical damage 
characteristic of this optical guided-wave device coupled to the optical 
fiber are as described above with reference to the second embodiment. 
In both the third and fourth embodiments, the heat treatment effect 
strengthening the bond results in a several fold increase in bond strength 
by simply baking the wafers for approximately one hour at 100.degree. C. 
This process yields a bond strength of several 10 kg/cm.sup.2. In general, 
bond strength increases with an increase in process temperature or time. 
When the temperature exceeds 1100.degree. C., however, lithium is rapidly 
released from the surface of both lithium niobate and lithium tantalate 
wafers, significantly deteriorating the wafer surface characteristics and 
optical guided-wave device performance. The bonding heat treatment 
temperature is therefore preferably less than 1100.degree. C. 
Because the thermal expansion coefficients are equivalent when two lithium 
niobate wafers are bonded as described in the third embodiment, the heat 
treatment temperature needed to improve bond strength is both higher and 
easier to regulate. In this case wafer separation does not occur even when 
a high mechanical polishing force is used to thin the wafer, and the 
optical guided-wave device itself can operate stably at a higher 
temperature. As a result, when the optical guided-wave device is 
manufactured by bonding wafers of like materials, a device with a high 
direct bond strength and stable operation at high temperatures can be 
obtained. 
This direct bonding is attributed to the ion bonding strength of the water, 
hydroxyl groups, and hydrogen adsorbed by the surface of the dielectric 
wafers from the water. Application of heat in this state causes the water 
to escape from the bonding interface and both directly adsorbed hydrogen 
and hydrogen in the hydroxyl groups to escape. The residual oxygen and 
oxygen in the surface of the dielectric (which is an oxide) to react with 
the other constituent elements of the dielectric, thereby increasing the 
bond strength. 
Fifth Embodiment 
This fifth embodiment describes a third embodiment of an optical 
guided-wave device structure according to the present invention is 
described below with reference to FIGS. 5 and 6. As in the first 
embodiment above, FIG. 5 is an oblique view of the optical guided-wave 
device as applied in an optical modulator, and FIG. 6 is a cross section 
of FIG. 5 at a section through the first and second optical wave guide 
branches 4 and 5. Identical parts in the first and third embodiments are 
identified by the same reference numerals 1-8. 
This embodiment differs from the first in that a glass film 21 bonds the 
lithium niobate substrate 1 to the lithium niobate thin-plate 2. 
The optical wave guide branches 4 and 5 and the I/O optical wave guide 3 
have a trapezoidal cross section characteristic of a so-called ridge-type 
optical wave guide. The guided light propagators 8 are located below the 
optical wave guide branches 4 and 5. The structure and function of the 
optical modulator are basically the same as described in the first 
embodiment. 
As also described in the first embodiment, the dielectric constant of 
lithium niobate parallel to the optical axis of the crystal is 
significantly different from the dielectric constant perpendicular to the 
optical axis, resulting in a corresponding difference in the refractive 
index in each direction. In this embodiment the crystal axes of the 
lithium niobate substrate 1 and the lithium niobate thin-plate 2 are 
selected so that the refractive index of the lithium niobate thin-plate 2 
is greater than that of the lithium niobate substrate 1 for the optical 
propagation mode. Although the refractive index of the bonding glass film 
21 between the wafers is approximately 1.5 and less than that of lithium 
niobate, light incident to the lithium niobate thin-plate 2 is trapped 
inside the thin-plate because the glass film 21 thickness is 0.5 .mu.m, 
significantly thinner than the optical wave guide blocking thickness. By 
additionally employing a ridge construction in the lithium niobate 
thin-plate 2, the effective dielectric constant of the area under the 
ridges is greater than that of the other (thinner) areas, thus trapping 
light below the ridges and enabling the under-ridge area to act as an 
optical wave guide. 
As in the first embodiment, the shape of the optical wave guide in this 
case is either trapezoidal or rectangular in end cross section with a 
uniform refractive index, thus placing the center of the guided light near 
the center of the optical wave guide with an approximately circular cross 
section. The cross section of the I/O optical wave guide 3 is also the 
same. This results in extremely good coupling efficiency between the round 
I/O optical wave guide structure and the core (approx. 10 .mu.m diameter) 
of the optical fiber. 
Typical values for these dimensions are a lithium niobate substrate 1 
thickness of 600 .mu.m, lithium niobate thin-plate 2 thickness of 7 .mu.m, 
peak ridge height of 3 .mu.m, optical wave guide width of 7 .mu.m, glass 
film thickness of 0.5 .mu.m, optical wave guide branch length of 2 cm, and 
total optical wave guide length of 3 cm. The electrodes are aluminum. 
With this construction, optical fiber coupling loss is less than 0.3 dB at 
one plane when coupled with adhesive bonding using an adhesive with an 
adjusted refractive index, a significant improvement over the coupling 
loss in conventional devices. The performance of the optical modulator 
itself is equivalent to that of the conventional titanium diffusion 
optical wave guide. 
Optical propagation loss of the optical wave guide is also significantly 
reduced because a lithium niobate thin-plate with the optical 
characteristics of pure single crystals is used without ion diffusion 
processing. Specifically, optical wave guide propagation loss of less than 
0.1 dB/cm is easily obtained. This is also a significant improvement over 
the conventional titanium diffusion optical wave guide in which 
propagation loss ranges from 0.5-1.0 dB/cm. 
The strength of the input light was also varied from 0 dBm to 30 dBm to 
determine the optical damage characteristic, but virtually no optical 
damage was observed. This is attributed to the use of a pure single 
crystal lithium niobate thin-plate with an extremely low level of trapped 
electrons as the optical wave guide. 
Measurements were made using a 1.3 .mu.m wavelength light. 
Sixth Embodiment 
This sixth embodiment describes a fourth embodiment of an optical 
guided-wave device structure according to the present invention as shown 
in FIGS. 7 and 8. FIG. 7 is an oblique view of the optical guided-wave 
device as applied in an optical modulator, and FIG. 8 is a cross section 
of FIG. 7 at a section through the first and second optical wave guide 
branches 4 and 5. This embodiment differs from the fifth in that a lithium 
niobate thin-plate 12 is bonded by a glass film 21 to a lithium tantalate 
substrate 11. Identical parts in the third and fourth embodiments are 
identified by the same reference numerals 3-8 and 21. 
As described in the second embodiment, the refractive indices of lithium 
tantalate and lithium niobate differ by an appropriate amount. Light can 
therefore be trapped in the lithium niobate layer with the higher 
refractive index, thus forming an optical wave guide. Light incident to 
the lithium niobate thin-plate 12 is therefore trapped inside the 
thin-plate. By additionally employing a ridge construction, the effective 
dielectric constant of the area under the ridges is greater than that of 
the other (thinner) areas, thus trapping light below the ridges and 
enabling the under-ridge area to act as an optical wave guide. 
The shape of the optical wave guide in this embodiment is identical to that 
of the fifth embodiment, and coupling efficiency with the round optical 
wave guide structure of the optical fiber is extremely good. 
Typical values for these dimensions are a lithium tantalate substrate 11 
thickness of 600 .mu.m with all other values the same as in the fifth 
embodiment above. 
With this construction, optical fiber coupling loss is less than 0.3 dB as 
in the fifth embodiment, a significant improvement over the conventional 
model. 
optical wave guide propagation loss of less than 0.1 dB/cm is easily 
obtained, again as in the fifth embodiment. The results of optical damage 
observations were also the same as in the fifth embodiment above. 
Seventh Embodiment 
This seventh embodiment describes a third embodiment of a manufacturing 
method for an optical guided-wave device according to the invention. 
First, the surfaces of two lithium niobate wafers with different crystal 
orientations and each mirror polished are cleaned with an etching process. 
On one face of each wafer, a lead borosilicate glass film is formed to a 
thickness of 0.25 .mu.m. The glass film sides of each wafer are then mated 
and heat is applied to bond the glass films together. 
The lithium niobate wafer with the crystal orientation having the higher 
refractive index is then mechanically polished and etched to a thin-plate 
layer. After reducing the wafer to a thickness of 7 .mu.m, an etching mask 
is formed on the thinned lithium niobate wafer to the pattern of the 
optical wave guide structure shown in the fifth embodiment using 
photolithography techniques, and the unmasked areas (the area not forming 
the optical wave guide) are removed to a depth of 3 .mu.m by etching. The 
mask is Cr and the etching agent is a hydrofluoric acid etching solution. 
The mask is then removed, and the aluminum electrodes are formed using 
conventional photolithography and etching technologies. 
This process yields the structure of the optical guided-wave device as 
shown in the fifth embodiment. The coupling characteristic, propagation 
loss, and optical damage characteristic of this optical guided-wave device 
and optical fiber are as described above with reference to the fifth 
embodiment. 
Bonding the two wafer using glass adhesion can be simply accomplished by 
setting the heat treatment temperature higher than the melting point of 
the glass, but adhesion can also be accomplished by maintaining a lower 
temperature near the softening point of the glass type used. Use of this 
lower temperature retains a glass film thickness approximately equal to 
the film thickness before heat treatment. While the film thickness was 
reduced, to varying degrees depending upon the pressure applied, with a 
heat treatment temperature exceeding the glass melting point, no adverse 
effects caused by the reduced thickness were observed. Glass compounds 
with a melting point ranging from 300.degree. C. to 800.degree. C. were 
used with good characteristics obtained in all cases by setting the heat 
treatment temperature at an appropriate level relative to the melting 
point, i.e., at a temperature above the softening point of the glass. 
Eight Embodiment 
This eighth embodiment describes a fourth embodiment of a manufacturing 
method for an optical guided-wave device according to the invention. 
As in the seventh embodiment above, the surfaces of a lithium niobate wafer 
and a lithium tantalate wafer, which are ground to a mirror finish, are 
cleaned with an etching process. The subsequent process is the same as in 
the seventh embodiment above, resulting in a glass bonded wafer of lithium 
tantalate and lithium niobate wafers. After reducing the thickness of the 
lithium niobate wafer, which has a higher refractive index, to 7 .mu.m, 
the wafer is masked, etched, and aluminum electrodes are formed as 
described in the seventh embodiment, resulting in an optical guided-wave 
device constructed as described in the sixth embodiment above. 
The coupling characteristic, propagation loss, and optical damage 
characteristic of this optical guided-wave device coupled to the optical 
fiber are as described above with reference to the sixth embodiment. 
In both the seventh and eighth embodiments, bond strength increases, in 
general, as the heat treatment process temperature or time increases. When 
the temperature exceeds 1100.degree. C., however, lithium is rapidly 
released from the surface of both lithium niobate and lithium tantalate 
wafers, significantly deteriorating the wafer surface characteristics and 
optical guided-wave device performance. The bonding heat treatment 
temperature is therefore preferably less than 1100.degree. C. 
Because the thermal expansion coefficients are equivalent when two lithium 
niobate wafers are bonded as described in the seventh embodiment, the heat 
treatment temperature needed to improve bond strength is both higher and 
easier to regulate. In this case wafer separation does not occur even when 
a high mechanical polishing force is used to thin the wafer, and the 
optical guided-wave device itself can operate stably at a higher 
temperature. As a result, when the optical guided-wave device is 
manufactured by bonding wafers of like materials, a device with a high 
direct bond strength and stable operation at high temperatures can be 
obtained. 
Ninth Embodiment 
This ninth embodiment describes a fifth embodiment of an optical 
guided-wave device structure according to the present invention is 
described below with reference to FIGS. 9 and 10. As in the first 
embodiment above, FIG. 9 is an oblique view of the optical guided-wave 
device as applied in an optical modulator, and FIG. 10 is a cross section 
of FIG. 9 at a section through the first and second optical wave guide 
branches 4 and 5. Identical parts in the first and third embodiments are 
identified by the same reference numerals 1-8. 
This embodiment differs from the fifth in that a silicon oxide film 22 
bonds the lithium niobate substrate 1 to the lithium niobate thin-plate 2. 
The optical wave guide branches 4 and 5 and the I/O optical wave guide 3 
have a trapezoidal cross section characteristic of a so-called ridge-type 
optical wave guide. The guided light propagators 8 are located below the 
optical wave guide branches 4 and 5. The structure and function of the 
optical modulator are basically the same as described in the first 
embodiment. 
As also described in the first embodiment, the dielectric constant of 
lithium niobate parallel to the optical axis of the crystal is 
significantly different from the dielectric constant perpendicular to the 
optical axis, resulting in a corresponding difference in the refractive 
index in each direction. In this embodiment the crystal axes of the 
lithium niobate substrate 1 and the lithium niobate thin-plate 2 are 
selected so that the refractive index of the lithium niobate thin-plate 2 
is greater than that of the lithium niobate substrate 1 for the optical 
propagation mode. As the refractive index of the bonding silicon oxide 
film 22 between the wafers is approximately 1.46 and less than that of 
lithium niobate, light incident to the lithium niobate thin-plate 2 is 
trapped inside the thin-plate. By additionally employing a ridge 
construction in the lithium niobate thin-plate 2, the effective dielectric 
constant of the area under the ridges is greater than that of the other 
(thinner) areas, thus trapping light below the ridges and enabling the 
under-ridge area to act as an optical wave guide. 
The shape of the optical wave guide in this case is either trapezoidal or 
rectangular in end cross section with a uniform refractive index, thus 
placing the center of the guided light near the center of the optical wave 
guide with an approximately circular cross section. The cross section of 
the I/O optical wave guide 3 is also the same. This results in extremely 
good coupling efficiency between the round I/O optical wave guide 
structure and the core (approx. 10 .mu.m diameter) of the optical fiber. 
Typical values for these dimensions are a lithium niobate substrate 1 
thickness of 600 .mu.m, lithium niobate thin-plate 2 thickness of 7 .mu.m, 
peak ridge height of 3 .mu.m, optical wave guide width of 7 .mu.m, silicon 
oxide film 22 thickness of 0.5 .mu.m, optical wave guide branch length of 
2 cm, and total optical wave guide length of 3 cm. The electrodes are 
aluminum. 
With this construction, optical fiber coupling loss is less than 0.3 dB at 
one plane when coupled with adhesive bonding using an adhesive with an 
adjusted refractive index, a significant improvement over the coupling 
loss in conventional devices. The performance of the optical modulator 
itself is equivalent to that of the conventional titanium diffusion 
optical wave guide. 
Optical propagation loss of the optical wave guide is also significantly 
reduced because a lithium niobate thin-plate with the optical 
characteristics of pure single crystals is used without ion diffusion 
processing. Specifically, optical wave guide propagation loss of less than 
0.1 dB/cm is easily obtained, a significant improvement over the 
conventional model. 
The strength of the input light was also varied from 0 dBm to 30 dBm to 
determine the optical damage characteristic, but virtually no optical 
damage was observed. This is attributed to the use of a pure single 
crystal lithium niobate thin-plate with an extremely low level of trapped 
electrons as the optical wave guide. 
Measurements were made using a 1.3 .mu.m wavelength light. 
Tenth Embodiment 
This tenth embodiment describes a sixth embodiment of an optical 
guided-wave device structure according to the present invention as shown 
in FIGS. 11 and 12. FIG. 11 is an oblique view of the optical guided-wave 
device as applied in an optical modulator, and FIG. 12 is a cross section 
of FIG. 11 at a section through the first and second optical wave guide 
branches 4 and 5. This embodiment differs from the ninth in that a lithium 
niobate thin-plate 12 is bonded by a silicon nitride film 23 to a lithium 
tantalate substrate 11. Identical parts in the first and tenth embodiments 
are identified by the same reference numerals 3-8. 
As described in the second embodiment, the refractive indices of lithium 
tantalate and lithium niobate differ by an appropriate amount. Light can 
therefore be trapped in the lithium niobate layer with the higher 
refractive index, thus forming an optical wave guide. Light incident to 
the lithium niobate thin-plate 12 is therefore trapped inside the 
thin-plate. By additionally employing a ridge construction, the effective 
dielectric constant of the area under the ridges is greater than that of 
the other (thinner) areas, thus trapping light below the ridges and 
enabling the under-ridge area to act as an optical wave guide. 
As the refractive index of the bonding silicon nitride film 23 between the 
wafers is approximately 1.9 and less than that of lithium niobate, light 
incident to the lithium niobate thin-plate 2 is trapped inside the 
thin-plate. 
The shape of the optical wave guide in this embodiment is identical to that 
of the ninth embodiment, and coupling efficiency with the round optical 
wave guide structure of the optical fiber is extremely good. 
Typical values for these dimensions are a silicon nitride film 23 thickness 
of 0.5 .mu.m, and a lithium tantalate substrate 11 thickness of 600 .mu.m 
with all other values the same as in the ninth embodiment above. 
With this construction, optical fiber coupling loss is less than 0.3 dB as 
in the ninth embodiment, a significant improvement over the conventional 
model. 
Propagation loss of less than 0.1 dB/cm is easily obtained, again as in the 
ninth embodiment. The results of optical damage observations were also the 
same as in the ninth embodiment above. 
Eleventh Embodiment 
This eleventh embodiment describes a seventh embodiment of an optical 
guided-wave device structure according to the present invention as shown 
in FIGS. 13 and 14. FIG. 13 is an oblique view of the optical guided-wave 
device as applied in an optical modulator, and FIG. 14 is a cross section 
of FIG. 13 at a section through the first and second optical wave guide 
branches 4 and 5. This embodiment differs from the tenth in that a lithium 
niobate thin-plate 12 is bonded by a silicon film 24 to a lithium 
tantalate substrate 11. Identical parts in the second and eleventh 
embodiments are identified by the same reference numerals 3-8. 
As described in the tenth embodiment, light incident to the lithium niobate 
thin-plate 12 is trapped inside the thin-plate because of the difference 
in the refractive indices of lithium tantalate and lithium niobate. By 
additionally employing a ridge construction, the effective dielectric 
constant of the area under the ridges is greater than that of the other 
(thinner) areas, thus trapping light below the ridges and enabling the 
under-ridge area to act as an optical wave guide. 
As the refractive index of the silicon film is less than that of the 
lithium niobate film and is absorbent to light with a 1.3 .mu.m 
wavelength, light incident to the lithium niobate thin-plate 2 is trapped 
inside the thin-plate. 
The shape of the optical wave guide in this embodiment is identical to that 
of the ninth embodiment, and coupling efficiency with the round optical 
wave guide structure of the optical fiber is extremely good. 
Typical values for these dimensions are a silicon film 24 thickness of 0.5 
.mu.m with all other values the same as in the ninth embodiment above. 
Optical fiber coupling loss is less than 0.3 dB as in the ninth 
embodiment, a significant improvement over the conventional model. 
Optical wave guide propagation loss of less than 0.1 dB/cm is easily 
obtained, again as in the ninth embodiment. The results of optical damage 
observations were also the same as in the ninth embodiment above. 
Twelfth Embodiment 
This twelfth embodiment describes a fifth embodiment of a manufacturing 
method for an optical guided-wave device according to the invention. 
First, the surfaces of two lithium niobate wafers with different crystal 
orientations and each mirror polished are cleaned with an etching process. 
A silicon oxide film is formed on one face of each wafer to a thickness of 
0.25 .mu.m using a plasma CVD method. The silicon oxide film surfaces are 
then cleaned with an etching process and treated with a hydrophilic 
process. Specifically, the silicon oxide film surface layer is etched to a 
very slight degree using a hydrofluoric acid etching solution, 
simultaneously cleaning the surface and making the surface hydrophilic. 
The surfaces are then flushed in demineralized water and immediately 
sandwiched uniformly together, easily enabling direct bonding by the 
water, hydroxyl groups, and hydrogen adsorbed in the silicon oxide film 
surface. While this process yields a sufficiently strong bond, the bond is 
further strengthened by heat treatment at a temperature between 
100.degree. C. and 1100.degree. C. 
The lithium niobate wafer with the crystal orientation having the higher 
refractive index is then mechanically polished and etched to a thin-plate 
layer. After reducing the wafer to a thickness of 7 .mu.m, an etching mask 
is formed on the thinned lithium niobate wafer to the pattern of the 
optical wave guide structure shown in the ninth embodiment using 
photolithography techniques, and the unmasked areas (the area not forming 
the optical wave guide) are removed to a depth of 3 .mu.m by etching. The 
mask is Cr. Etching is performed by reactive ion etching using a CF.sub.4 
gas. The mask is then removed, and the aluminum electrodes are formed 
using conventional photolithography and etching technologies. 
This process yields the structure of the optical guided-wave device as 
shown in the ninth embodiment. The coupling characteristic, propagation 
loss, and optical damage characteristic of this optical guided-wave device 
and optical fiber are as described above with reference to the ninth 
embodiment. 
Heat treatment of the silicon oxide film is possible in the range from 
100.degree. C. to 1100.degree. C. with a higher heat treatment temperature 
yielding a higher bond strength. 
Thirteenth Embodiment 
This thirteenth embodiment describes another manufacturing method for an 
optical guided-wave device according to the invention. 
As in the twelfth embodiment above, the surfaces of a lithium niobate wafer 
and a lithium tantalate wafer, which are ground to a mirror finish, are 
cleaned with an etching process. A silicon oxide film is formed on one 
face of each wafer to a thickness of 0.25 .mu.m using a plasma CVD method. 
As in the twelfth embodiment, the silicon oxide film surfaces are then 
cleaned with an etching process and treated with a hydrophilic process, 
flushed in demineralized water, and immediately sandwiched uniformly 
together, thereby bonding the lithium tantalate and lithium niobate layers 
together by means of the silicon oxide film. Bond strength is increased by 
heat treatment at 100.degree.-1100.degree. C. 
The subsequent process is the same as in the twelfth embodiment above, 
resulting in silicon oxide film bonding of the lithium tantalate and 
lithium niobate wafers. Aluminum electrodes are then formed as described 
in the twelfth embodiment, resulting in an optical guided-wave device 
constructed as described in the tenth embodiment above. 
The coupling characteristic, propagation loss, and optical damage 
characteristic of this optical guided-wave device and optical fiber are as 
described above with reference to the tenth embodiment. 
Heat treatment of the silicon oxide film is possible in the range from 
100.degree. C. to 1100.degree. C. with a higher heat treatment temperature 
yielding a higher bond strength. 
Fourteenth Embodiment 
This fourteenth embodiment describes a seventh embodiment of the 
manufacturing method for an optical guided-wave device according to the 
invention. 
As in the twelfth embodiment above, the surfaces of a lithium niobate wafer 
and a lithium tantalate wafer, which are ground to a mirror finish, are 
cleaned with an etching process. An amorphous silicon film is formed on 
one face of each wafer to a thickness of 0.25 .mu.m using a plasma CVD 
method. As in the twelfth embodiment, the amorphous silicon film surfaces 
are then cleaned with an etching process and treated with a hydrophilic 
process, flushed in demineralized water, and immediately sandwiched 
uniformly together, thereby bonding the lithium tantalate and lithium 
niobate layers together by means of the amorphous silicon film. Bond 
strength is increased by heat treatment at 100.degree.-1100.degree. C. 
The subsequent process is the same as in the twelfth embodiment above, 
resulting in amorphous silicon film bonding of the lithium tantalate and 
lithium niobate wafers. Aluminum electrodes are then formed as described 
in the twelfth embodiment, resulting in an optical guided-wave device 
constructed as described in the eleventh embodiment above. 
The coupling characteristic, propagation loss, and optical damage 
characteristic of this optical guided-wave device and optical fiber are as 
described above with reference to the eleventh embodiment. 
Heat treatment of the amorphous silicon film is possible in the range from 
100.degree. C. to 1100.degree. C. with a higher heat treatment temperature 
yielding a higher bond strength. However, if the heat treatment 
temperature exceeds the crystallization temperature of the amorphous 
silicon, the amorphous silicon changes to a polycrystalline silicon film 
but the bond state is maintained. 
In the twelfth, thirteenth, and fourteenth embodiments described above, 
bond strength increases, in general, as the heat treatment temperature 
increases. When the temperature exceeds 1100.degree. C. however lithium is 
rapidly released from the surface of both lithium niobate and lithium 
tantalate wafers, significantly deteriorating the wafer surface 
characteristics and optical guided-wave device performance. The bonding 
heat treatment temperature is therefore preferably less than 1100.degree. 
C. 
Because the thermal expansion coefficients are equivalent when two lithium 
niobate wafers are bonded as described in the twelfth embodiment, the heat 
treatment temperature needed to improve bond strength is both higher and 
easier to regulate. In this case wafer separation does not occur even when 
a high mechanical polishing force is used to thin the wafer, and the 
optical guided-wave device itself can operate stably at a higher 
temperature. As a result, when the optical guided-wave device is 
manufactured by bonding wafers of like materials, a device with a high 
bond strength and stable operation at high temperatures can be obtained. 
The thickness of the bonding film in these three embodiments can also be 
freely controlled within the range from 0.1 to 3 .mu.m by changing the 
plasma CVD conditions. 
Direct bonding is attributed to the ion bonding strength of the water, 
hydroxyl groups, and hydrogen adsorbed by the surface of the silicon 
oxide, silicon nitride, and amorphous silicon layers. Application of heat 
in this state causes the water to escape from the bonding interface and 
both directly adsorbed hydrogen and hydrogen in the hydroxyl groups to 
escape. The residual oxygen and oxygen in the surface of the dielectric 
(which is an oxide) to react with the other constituent elements of the 
dielectric, thereby increasing the bond strength. 
It is to be noted that while typical dimensions are described for each of 
the above embodiments, the invention shall not be so limited to the stated 
dimensions and any ranges of values enabling formation of a good optical 
wave guide are within the scope of the invention. 
In addition, lithium niobate and lithium tantalate are used by way of 
example only as single crystal dielectric bodies. It will be obvious that 
the principle of the invention is applicable to other dielectric bodies 
having an electro-optic effect and formed in a similar manner. 
The preferred embodiments above describe only examples of the relationship 
between specific wafer types and bonding films, and various other 
combinations are also valid. For example, in the twelfth embodiment above, 
a silicon nitride film or silicon film can be substituted for the silicon 
oxide film, and a silicon oxide film can be used in the thirteenth 
embodiment while obtaining an equivalent structure and effect. When using 
a silicon film, the initial film state can be either amorphous or 
polycrystalline, and the state obtained after heat treatment and dependent 
upon the heat treatment temperature can be either amorphous or 
polycrystalline. 
It is also to be noted that while the above embodiments are described as 
applied by way of example in a Mach-Zehnder type optical modulator, the 
structure of the optical modulator shall not be so limited and the 
principle of the invention is equally applicable to other structures using 
an optical wave guide. It is also obvious that the optical modulator can 
used as an optical switch if the guided light is modulated by the 
structure of the optical modulator in a switching manner. The present 
invention also applies to optical guided-wave devices which control the 
guided light using an electro-optic effect by applying a voltage to the 
optical wave guide. Examples of such devices include plane of polarization 
control devices, optical phase matching devices and optical propagation 
mode control devices. 
The structure shown in the first embodiment is effective as a mode splitter 
for splitting light by controlling the propagation mode of the propagated 
light because the refractive indexes to ordinary and extraordinary light 
of the optical wave guide and the substrate differ. 
[Effect of the Invention] 
The following effects are achieved by means of the structure and method of 
the invention as described above. 
With specific regard to the optical wave guide, the symmetry of the shape 
of the optical wave guide in cross section is good, and the center of 
light propagation is positioned at approximately the center of the 
thin-plate because of the uniform thickness of the device layers, and the 
layer thickness can be freely controlled. Coupling loss with the optical 
fiber is therefore greatly reduced. 
Because there is also a great degree of freedom in the selection of 
materials used for the optical wave guide, and it is therefore possible to 
use pure single crystal dielectric wafers which have not been subject to 
diffusion processing, an optical guided-wave device with low optical 
propagation loss and minimal optical damage can be obtained. 
Wafer processing is also simpler and a device with stable characteristics 
to high temperature levels can be obtained when the bonded wafers are of 
like materials because the equivalent thermal expansion coefficients of 
the materials make it easier to use a high temperature in the heat 
treatment used to improve the bond strength. 
Finally, while the preferred embodiments are described as applied in an 
optical modulator, the essential feature of the invention is the structure 
of the optical wave guide itself. As a result, the invention can be 
generally applied in a wide range of optical guided-wave devices using an 
optical wave guide, and the invention can be also applied in optical 
switches, plane of polarization controllers, propagation mode controllers, 
and other optical guided-wave devices other than optical modulators. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.