Optical waveguide, optical module and optical system using the same

An optical waveguide comprises an anhydrous synthetic silica glass substrate 1, which contains less than a predetermined concentration of hydroxyl group, a core waveguide 2 having a rectangular cross-section being formed on the substrate 1, and a cladding layer 3 having a lower refractive index thereof than the core waveguide 2 being covered by the cladding layer 3. The concentration of hydroxyl group contained in a synthetic silica glass substrate of an optical waveguide or an optical device is essentially less than 300 ppm, preferably less than 100 ppm, more preferably less than 50 ppm.

BACKGROUND OF THE INVENTION 
This invention relates to an optical waveguide, an optical module, and an 
optical system using the same, and more particularly to, an optical 
waveguide, an optical module, and an optical system using the same which 
are suitable for optical circuit devices, such as optical star couplers, 
optical multiplexers/demultiplexers, optical switches, optical modulators, 
wavelength-independent optical couplers, etc., and optical transmission 
systems. 
At present, many optical systems, such as an optical subscriber's system, 
an optical CATV, an optical submarine cable transmission system and an 
optical information processing system, have been actively developed. For 
the configuration of such optical systems, optical circuit devices, such 
as optical star couplers, optical multiplexers/demultiplexers, optical 
switches, optical modulators, wavelength-independent optical couplers, and 
optical transmission modules, in which such optical circuit devices such 
as a semiconductor laser and/or a photodiode, etc. are incorporated, are 
essential devices. An optical fiber-type device and an optical 
waveguide-type device are known to be used for such optical circuit 
devices. The optical waveguide-type device is expected to achieve a small 
size, low-cost and productive device because it also has the functions of 
the optical fiber-type device. 
An optical waveguide using a semiconductor substrate like silicon and an 
optical waveguide using a silica glass substrate are already known. 
However, the optical waveguide using the silica glass substrate is more 
advantageous because it can be connected to an optical fiber by a fusing 
technique and less polarization-dependent loss is obtained. 
A conventional optical waveguide comprises a silica glass substrate, at 
least one core waveguide formed thereon, and a cladding layer covering the 
core waveguide, wherein a certain amount of at least one dopant is added 
to both the core waveguide and the cladding layer so that the refractive 
index of the core waveguide is higher than that of the cladding layer. For 
fabricating the conventional optical waveguide, a silica glass substrate 
wafer is prepared, and a doped SiO.sub.2 glass layer is deposited by 
electron-beam deposition, which doped SiO.sub.2 glass layer is finally 
formed into a core waveguide. Next, a metal mask is formed on the doped 
SiO.sub.2 glass layer by sputtering, and a photoresist layer is formed on 
the metal mask by photolithography. After that, a core waveguide is 
patterned on the substrate by reactive-ion etching. At this step, the 
substrate is treated at a high temperature of more than 1200.degree. C. in 
order to stabilize the refractive index of the core waveguide. Next, a 
SiO.sub.2 porous glass layer as a cladding layer is formed by flame 
deposition by hydrolyzing source gases, then heated and consolidated at 
more than 1200.degree. C., and the cladding layer of transparent glass is 
obtained. Finally, the wafer is diced into a plurality of optical 
waveguides by a blade. 
In the conventional optical waveguide and the conventional optical device 
using the same, however, there are disadvantages in that its connecting 
loss is likely to be extremely high, therefore the yield of fabrication 
thereof is low. Deformation of the substrate occurs by the high 
temperature treatment in the fabrication process, which causes a 
difference in the axes between the core waveguide and an optical fiber. 
Such deformations vary not only along the optical waveguide but also on 
the plane of the silica glass wafer where the optical waveguides are 
formed, and wafer by wafer. 
Another disadvantage, is that the expected optical characteristics of 
optical devices, such as optical multiplexers/demultiplexer and 
wavelength-independent optical couplers are not obtained, because they 
depend on the lengths of the core waveguides. Furthermore, there is a 
disadvantage in that an absorption loss at a wavelength of 1.39 .mu.m 
exists, which seems to be caused by the existence of hydroxyl groups. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an optical 
waveguide, an optical module, and an optical system using the same to 
which an optical fiber can be connected precisely, so that alignment of 
the axes between the core waveguide and the optical fiber is realized. 
It is a further object of the invention to provide an optical waveguide, an 
optical module, and an optical system using the same by which expected 
optical characteristics are obtained without special treatment, and 
absorption loss at a wavelength of 1.39 .mu.m can be reduced. 
It is a still further object of the invention to provide an optical 
waveguide, an optical module, and an optical system using the same which 
can be fabricated with high yield and low cost. 
According to the first feature of the invention, an optical waveguide 
comprises: a substrate; and at least one core waveguide formed on the 
substrate for transmitting light therein; wherein the substrate is a pure 
SiO.sub.2 synthetic silica glass substrate containing less than a 
predetermined concentration of hydroxyl groups. 
According to the second feature of the invention, an optical module 
comprises: a substrate; at least one core waveguide formed on the 
substrate for transmitting light therein; at least one optical device for 
emitting light into the at least one core waveguide or receiving light 
from the at least one core waveguide, the at least one optical device 
being connected to one end of the at least one core waveguide; and at 
least one optical fiber connected to an opposite end of the one core 
waveguide; wherein the substrate is a pure SiO.sub.2 synthetic silica 
glass substrate containing less than a predetermined concentration of 
hydroxyl groups. 
According to the third feature of the invention, an optical system 
comprises: an optical emitting module comprising: a substrate; at least 
one core waveguide formed on the substrate for transmitting light therein; 
at least one optical device for emitting light into the at least one core 
waveguide, the at least one optical device being connected to one end of 
the at least one core waveguide; wherein the substrate is a pure SiO.sub.2 
synthetic silica glass substrate containing less than a predetermined 
concentration of hydroxyl groups; an optical receiving module comprising: 
a substrate; at least one core waveguide formed on the substrate for 
transmitting light therein; at least one optical device for emitting light 
into the at least one core waveguide or receiving light from the at least 
one core waveguide, the at least one optical device being connected to one 
end of the at least one core waveguide; wherein the substrate is a pure 
SiO.sub.2 synthetic silica glass substrate containing less than a 
predetermined concentration of hydroxyl groups; an optical transmitter in 
which the optical emitting module is incorporated; an optical receiver in 
which the optical receiving module is incorporated; and an optical fiber 
for connecting the optical transmitter to the optical receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before explaining an optical waveguide and optical device using the same in 
the first preferred embodiment, the aforementioned conventional connecting 
structures will be explained in FIGS. 1 and 2. 
FIG. 1 shows a conventional optical waveguide, which comprises a silica 
glass substrate 41, at least one core waveguide 42 (two core waveguides 
are shown in FIG. 1) formed on the silica glass substrate 41, and a 
cladding layer 43 covering the core waveguide 42, wherein a certain amount 
of at least one dopant is added into both the core waveguide 42 and the 
cladding layer 43 so that the refractive index of the core waveguide 42 is 
higher than that of the cladding layer 43. 
FIG. 2 shows the conventional method for fabricating the optical waveguide 
shown in FIG. 1. First, a silica glass substrate wafer 51 is prepared, and 
a doped SiO.sub.2 glass layer 54 is deposited by electron-beam deposition, 
which layer 54 is finally formed into a core waveguide (STEP A). Next, a 
metal mask 55 is formed on the doped SiO.sub.2 glass layer 54 by 
sputtering (STEP B), and a photoresist layer 56 is formed on the metal 
mask 55 by photolithography (STEP C). After that, a core waveguide 52 is 
patterned on the substrate 51 by means of a reactive-ion etching (STEP D). 
At this step, the substrate 51 is treated at a high temperature of more 
than 1200.degree. C. in order to stabilize the refractive index of the 
core waveguide 52. Next, a SiO.sub.2 porous glass layer 53 as a cladding 
layer is formed by flame deposition by hydrolyzing source gases (STEP E), 
then heated and consolidated at more than 1200.degree. C., whereby the 
cladding layer 53 of transparent glass is obtained (STEP F). Finally, the 
wafer 51 is diced into a plurality of optical waveguides 58 by a blade 59 
(STEP G). 
A conventional 2-input.times.16-output waveguide-type optical star coupler 
(not shown) was fabricated by the method shown in FIG. 2. Optical fiber 
arrays (not shown), one ends of which were positioned in a V-shaped groove 
(not shown) formed on the surface of a block (not shown), were connected 
to the output ports of the optical star coupler. However, a certain number 
of output ports had extremely high connecting losses, and the fabrication 
yield was low. The cause thereof was investigated by the inventors, and it 
was found that deformation of the substrate occurs during the high 
temperature treatment in the fabrication process, which causes the 
difference in the axes between the core waveguide and the optical fiber. 
For example, such deformation includes the contraction of the pitches 
between each core waveguide, and large warps of the substrates make it 
almost impossible to mount on a package. As such deformations vary 
depending on not only the optical waveguide itself but also the plane of 
the silica glass wafer where the optical waveguides are formed and each 
wafer, it was difficult to eliminate these disadvantages by designing a 
optimum configuration of the waveguide including such contraction and 
warps which would occur. 
The deformations discussed above also caused difficulty in obtaining 
expected optical characteristics of optical devices, in optical circuits 
such as optical multiplexer/demultiplexers and wavelength-independent 
optical couplers, because the optical characteristics of such circuits 
depend on the lengths of the core waveguides. 
Furthermore, an absorption loss at a wavelength of 1.39 .mu.m existed in 
the conventional optical waveguide and the conventional optical device 
using the same. This absorption loss seems to be caused by the existence 
of hydroxyl groups. The inventors considered it was caused by the 
condition of the core waveguide layer forming process, and studied the 
various forming conditions. However, the absorption losses could not be 
eliminated. 
Next, referring to FIG. 3, a principle of an optical waveguide according to 
the invention will be explained. The optical waveguide comprises an 
anhydrous silica glass substrate 1, at least one core waveguide 2 in which 
a light travels, and a cladding layer 3 covering the core waveguide 2. The 
inventors have found that the lower the concentration of hydroxyl groups 
is, the higher the heat-resistant temperature is in a pure-SiO.sub.2 
synthetic silica glass substrate. If such pure SiO.sub.2 synthetic silica 
glass substrate which contains less than a predetermined concentration of 
hydroxyl groups is used, contraction and warping of the waveguide become 
much lower. Therefore, optical fibers can be precisely connected to the 
core waveguides of the optical waveguide, and the expected optical 
characteristics are obtained as designed. Furthermore, the inventors has 
found that the optical absorption at 1.39 .mu.m is caused by the existence 
of hydroxyl groups in the core waveguide which are diffused from the 
silica glass substrate. Therefore, if the silica glass substrate is 
substantially free from hydroxyl groups, the optical absorption can be 
reduced. 
In the invention, the concentration of hydroxyl groups contained in a 
synthetic silica glass substrate of an optical waveguide or an optical 
device is essentially less than 300 ppm, preferably less than 100 ppm, 
more preferably less than 50 ppm. 
For fabricating synthetic silica glasses, it is known that silicon 
tetrachloride is decomposed in a oxyhydrogen flame or plasma flame to 
deposit silica glass. Also, for reducing the amount of hydroxyl groups in 
synthetic silica glasses, dehydration by chlorine is known. However, both 
process yield residual chloride of 10 ppm .about.1000 ppm in silica 
glasses. This residual chloride may make the heat resistant 
characteristics of the glass, i.e. the softening temperature of the glass, 
lower. Therefore, it is also important for the invention that a synthetic 
silica glass substrate is substantially free from chlorine. 
Furthermore, the synthetic silica glass substrate needs to have a 
deformation temperature (which is defined below) of 1000.degree. C., 
preferably more than 1050.degree. C. The deformation temperature is 
defined as a temperature where the coefficient of viscosity is 10.sup.14.5 
P (poise), which is determined by setting and heating a wafer having a 6 
inch diameter and 0.8 mm thickness in a furnace, placing a weight of 500 g 
(gram) thereon, and measuring the extension of the wafer at each 
temperature. The less the hydroxyl group concentration is, the higher the 
deformation temperature becomes, and less deformation of the substrate 
occurs. 
In the invention, a pure SiO.sub.2 synthetic glass substrate means a 
SiO.sub.2 synthetic glass substrate which contains less than 10 ppm of 
heavy metal impurities like Fe, Cu, alkali metals like Na, K, etc. and 
alkali earth metals like Ca. These metal impurities may lower the 
deformation temperature or the softening temperature, and may affect the 
refractive index of the core waveguide by diffusion thereto. Therefore, 
the concentration of the metal impurities must be less than 10 ppm, 
preferably 1 ppm. 
Next, an optical waveguide in the preferred embodiment according to the 
invention will be explained with reference to FIG. 3. The optical 
waveguide comprises an anhydrous synthetic silica glass substrate 1, which 
contains less than a predetermined concentration of hydroxyl groups, a 
core waveguide 2 having a rectangular cross-section being formed on the 
substrate 1, and a cladding layer 3 having a lower refractive index than 
the core waveguide 2 and which covers the cladding layer 3. For example, 
the core waveguide is 8 .mu.m wide and 8 .mu.m high, and made of TiO, 
--SiO.sub.2 glass, etc. The material as the cladding layer may be B.sub.2 
O.sub.3 --P.sub.2 O.sub.5 --SiO.sub.2 glass, for example, and the 
composition thereof is determined so that the refractive index difference 
between the core waveguide and the cladding layer is approximately 0.3%. 
Conventional hydroxyl group containing synthetic silica glass wafers, the 
deformation of which occurs during high temperature treatment in their 
fabrication process, were prepared and their hydroxyl group concentrations 
were measured. As a result, each wafer had a hydroxyl group concentration 
of approximately 1000 ppm, and minimum values thereof in part of each 
wafer surface were more than 400 ppm. 
The inventors considered that the concentration of hydroxyl groups in the 
wafer (substrate) is associated with the heat-resistant characteristics, 
and experimented on wafers having lower concentrations of hydroxyl groups. 
As a result, it has found that the lower the concentration of hydroxyl 
groups is, the less deformation of the wafer, i.e., contraction and warp, 
occurs during high temperature treatment in the fabrication process. 
Because the substrate becomes more difficult to soften at high 
temperature, that means an improvement in heat-resistance by decrease of 
hydroxyl group concentration. 
Experiments were carried out as follows. Ten (10) wafers each of 
hydroxyl-containing synthetic silica glass wafers and anhydrous synthetic 
silica glass wafers were prepared and seven (7) devices of 2-input 
16-output waveguide-type optical star couplers were produced from each 
wafer. In this case, the total width of the 16-output port was 5 mm. As a 
result, regarding contractions, the devices made from the 
hydroxyl-containing synthetic silica wafers had a mean contraction value 
of more than about 4.5 .mu.m of the pitch from the designed value, and 
they varied depending on each wafer and the location within the wafer. By 
contrast, the devices made from the anhydrous synthetic silica wafers had 
a mean contraction value of less than 2 .mu.m, even if the concentration 
of hydroxyl groups is between more than 200 ppm and less than 300 ppm. The 
contraction values were suppressed to less than 1 .mu.m with a 
concentration of hydroxyl groups of 100 ppm, and to 0.5 .mu.m with 50 ppm. 
Regarding warps of the wafers, the devices made from the hydroxyl 
containing synthetic silica glass wafers not only had a mean value of 1 
.mu.m, but also ten percent of these devices had warps of more than 2 
.mu.m. Devices having such large warps are considered to be inappropriate 
for mounting on a package. By contrast, the devices made from the 
anhydrous synthetic silica glass wafers had a mean warp of less than 0.8 
.mu.m, even if the concentration of hydroxyl group is between more than 
200 ppm and less than 300 ppm. The warps were reduced to less than 0.5 
.mu.m with a concentration of hydroxyl groups of 100 ppm, and to 0.2 .mu.m 
with 50 ppm. As indicated above, in order to reduce a deformation of an 
optical waveguide or an optical device, the concentration of hydroxyl 
groups contained in the synthetic silica glass substrate of the optical 
waveguide or the optical device is essentially less than 300 ppm, 
preferably less than 100 ppm, more preferably less than 50 ppm. 
Next, a Mach-Zehnder interferometer shown in FIG. 6, and a wavelength 
division multiplexer/demultiplexer (WDM) filter shown in FIG. 7 and made 
according to the invention will be demonstrated as follows. 
FIG. 6 shows the Mach-Zehnder interferometer 60, which comprises a first 
core waveguide 61 and a second core waveguide 62, which are adjacent to 
each other at two spaced apart positions to form couplers 63, 64, wherein 
the lengths of the first core waveguide and the second core waveguide 
between the two couplers are designed to be different. The Mach-Zehnder 
interferometer is used for a long distance optical transmission system and 
an optical amplifier for a fiber sensing system, etc. In operation, for 
example, when it is used as an optical amplifier, a superposed input light 
66, which includes an excitation light of 0.98 .mu.m and a signal light of 
1.55 .mu.m and is output from an amplifying medium, is input into an input 
end of the first core waveguide 61, then the signal light of 1.55 .mu.m, 
and the excitation light of 0.98 .mu.m are divided and output from output 
ends of the first core waveguide and the second core waveguide as output 
lights 67, respectively. FIG. 7 shows the WDM filter, which comprises an 
input waveguide 71, a plurality of output waveguides 72 (8 output 
waveguides in FIG. 7) and arrayed waveguides 75 each having different 
lengths, both ends of which are connected to an output end of the input 
waveguide 71 and an input end of the output waveguide 72 by slab 
waveguides 73, 74, respectively. The WDM filter 70 may be used as a part 
of a light source section of an optical transmitter or a detecting section 
of a receiver in a wavelength-division multiplexing optical transmission 
system. In operation, input signal lights of .lambda.1.about..lambda.8, 
which are input from the input waveguide 71, are divided into each signal 
light and output from each output waveguide 72. Otherwise, input signal 
lights of .lambda.1.about..lambda.8, which are input from each output 
waveguide 72, respectively, are multiplexed and output from the input 
waveguide 71. 
Experiments were carried out with those Mach-Zehnder interferometers shown 
in FIG. 6 and WDM filters shown in FIG. 7, which were made from both 
hydroxyl-containing synthetic silica glass wafers (substrates) and 
anhydrous synthetic silica wafers (substrates). As a result, each device 
made from hydroxyl group containing synthetic silica glass wafers 
(substrates) contracted in its circuit size, and less than 30% of samples 
met the expected optical characteristics. By contrast, each device made 
from anhydrous synthetic silica glass wafers (substrates) did not contract 
as much, and 85% of samples having a hydroxyl group concentration of less 
than 300 ppm met the expected optical characteristics. Also, more than 95% 
of samples having less than 100 ppm hydroxyl group, and almost 100% having 
less than 50 ppm met the expected optical characteristics. Therefore, in 
order to improve the optical characteristics of an optical device, as 
mentioned before, the concentration of hydroxyl groups contained in a 
synthetic silica glass substrate of an optical device is essentially less 
than 300 ppm, preferably less than 100 ppm, more preferably less than 50 
ppm. 
For fabricating a synthetic silica glass substrate, if silicon 
tetrachloride is used as a source gas, or if the dehydration process is 
conducted in a chlorine-containing gas atmosphere, a certain amount of 
chlorine remains in the substrate or is absorbed into the substrate. Such 
residual chlorine may cause a reduction in softening temperature, i.e. 
deformation temperature. In the embodiment according to the invention, 
such source gasses that do not contain chlorine, such as methoxysilane, 
etc., are used for the deposition of synthetic silica glass, and the 
dehydration process is carried out in vacuum or inert gas atmosphere 
excluding chlorine, so that the substrate does not substantially contain 
chlorine. 
Moreover, as described before, according to the invention, a pure SiO.sub.2 
synthetic glass substrate means a SiO.sub.2 synthetic glass substrate 
which contains less than 10 ppm of metal impurities of heavy metals like 
Fe, Cu, alkali metals like Na, K, etc. and alkali earth metals like Ca. 
These metal impurities may lower the deformation temperature or the 
softening temperature, and may affect the refractive index of the core 
waveguide by diffusion. Therefore, in order to avoid such problems, 
high-purity source gases, in which metal impurities of less than 10 ppm, 
preferably 1 ppm, are contained, are used for the invention. 
FIG. 4 shows the dependence of deformation temperature upon hydroxyl group 
concentration. As described before, the deformation temperature is defined 
as the temperature where the coefficient of viscosity is 10.sup.14.5 p 
(poise) which is determined by setting and heating a wafer having 6 inch 
diameter and 0.8 mm thickness in a furnace, placing a weight of 500 g 
(gram) thereon, and measuring the extension of the wafer at each 
temperature. The graph indicates that the conventional substrate of 
hydroxyl group containing synthetic silica, which contains hydroxyl groups 
of more than 400 ppm, has a deformation temperature of less than 
1000.degree. C. By contrast, the substrate of anhydrous synthetic silica, 
which contains hydroxyl groups of less than 100 ppm, has a deformation 
temperature of more than 1050.degree. C. This result shows that the 
concentration of hydroxyl groups affects the deformation temperature, i.e. 
heat-resisting characteristics. Therefore, in order to reduce deformation 
of the substrate due to high temperature treatment in its fabrication 
process, the deformation temperature is to be greater than 1000.degree. 
C., preferably 1050.degree. C., more preferably 1100.degree.C. (the 
concentration of hydroxyl groups being less than 10 ppm). The natural 
silica (flame-fused) glass substrate has higher deformation temperatures 
than that of anhydrous synthetic silica glass substrate. However, it 
contains metal impurities of more than several tens of ppm, which may 
deleteriously affect the optical characteristics of the optical device. 
For example, such impurities diffuse into a core waveguide on the 
substrate and vary the refractive index thereof. Therefore, this substrate 
is rather inappropriate for a silica glass substrate of the invention. 
Next, in order to investigate the influence of hydroxyl groups on optical 
absorption at 1.39 .mu.m in a core waveguide, some experiments were 
carried out by the inventors. Substrates of both hydroxyl group containing 
synthetic silica glass and anhydrous synthetic silica glass were prepared, 
and optical waveguides having a 8 .mu.m.times.8 .mu.m rectangular cross 
section, 6 cm long straight core waveguide were fabricated and the 
absorption loss characteristics as a function of wavelength were measured. 
In this case, the hydroxyl group containing synthetic silica glass 
substrate contains hydroxyl groups of 420 ppm, and the anhydrous synthetic 
silica glass substrate contains 50 ppm hydroxyl groups, respectively. The 
results are shown in FIGS. 5A and 5B. Although in the hydroxyl 
group-containing synthetic silica glass substrate, as shown in FIG. 5A, a 
peak absorption loss of more than 1 dB is observed at 1.39 .mu.m 
wavelength, there is no peak in the anhydrous synthetic silica glass 
substrate, as shown in FIG. 5B. Therefore, in order to reduce the 
absorption loss at 1.39 .mu.m in the core waveguide, it is necessary to 
decrease the concentration of hydroxyl groups in the substrate as well as 
to take conditions of the fabrication process of the core waveguide into 
consideration. 
In the invention, each value of the concentration of hydroxyl groups was 
measured by an infrared spectroscopic analyzer (made by Nippon Bunko Sha). 
In the invention, an optical module, which comprises an optical waveguide 
made from an anhydrous synthetic silica glass substrate as explained 
above, an optical element such as a light emitting element, light 
detector, connected to one end of a core waveguide, and an optical fiber 
connected to another end thereof, may be structured. In this case, any 
defect in the connection of the core waveguide to such elements due to 
deformation of the optical waveguide does not occur, so that improved 
productivity and low cost of the optical module are realized. Furthermore, 
optical systems such as optical transmitters and optical receivers with 
high reliability may be realized by using such an optical module. 
Moreover, as explained before, an optical system, which comprises an 
optical fiber, an optical circuit device, such as an optical star coupler, 
optical multiplexers/demultiplexers, optical switches, optical modulators, 
wavelength-independent optical couplers, etc., and connected to the 
optical circuit device, the optical fiber being connected to both an input 
port and an output port of the optical circuit device, may be structured. 
As the optical circuit device has such improved optical characteristics, 
and the optical fibers may be connected to the optical circuit device so 
precisely, an optical system with high reliability may be realized. 
As explained above, the invention provides the advantages set out below: 
(1) an optical waveguide has little substrate deformation during its 
fabrication process. Therefore, the optical waveguide can be connected to 
an optical fiber without differences of axes, and expected optical 
characteristics can be realized. 
(2) much less amounts of hydroxyl groups diffuse from the substrate into a 
core waveguide, so that optical absorption at 1.39 .mu.m can be reduced. 
(3) improved productivity and low cost of an optical module are realized. 
Therefore, optical systems such as optical transmitters and optical 
receivers with high reliability may be realized by using such optical 
module. 
(4) an optical circuit device has improved optical characteristics, and 
optical fibers may be connected to the optical circuit device precisely, 
so that an optical system with high reliability may be realized. 
Although the invention has been described with respect to a specific 
embodiment for complete and clear disclosure, the appended claims are not 
to be thus limited but are to be construed as embodying all modifications 
and alternative constructions that may occur to one skilled in the art and 
which fairly fall within the basic teaching herein set forth.