Two-way optical communication device and two-way optical communication apparatus

A two-way optical communication module includes a main waveguide which is optically coupled to an optical fiber and directs light incident from the optical fiber to a light-receiving element and a sub waveguide which is optically coupled to the side of the main waveguide and directs light emitted from a light-emitting element to the optical fiber so as to carry out an efficient two-way optical communication by using one optical fiber without the necessity for an optical branch device. With this arrangement, it is possible to provide an inexpensive and small-sized two-way optical communication module which readily enables an integration with other elements so as to exhibit suitability for a small-scale network, and to provide a two-way optical communication link which uses the two-way optical communication module.

FIELD OF THE INVENTION 
This invention relates to a two-way optical communication device which is 
capable of two-way transmission and reception of an optical signal, more 
specifically, the two-way communication device for home communication, 
communication between electronic devices, a LAN(Local Area Network), etc. 
that adopts a multimode optical fiber such as a plastic optical fiber as a 
transmitting medium, and this invention further concerns a two-way optical 
communication apparatus which uses the two-way communication device. 
BACKGROUND OF THE INVENTION 
Conventionally, as a two-way optical communication apparatus, the following 
two-way optical communication links have been known: a single-mode optical 
fiber which transmits single-mode light and serves as a transmitting 
medium, and a multimode optical fiber which transmits multimode light and 
serves as a transmitting medium. 
An example of the single-mode optical fiber is a quartz glass optical fiber 
whose core is made of quartz glass. A loss caused by the quartz glass 
optical fiber is so small that transmission is possible over a long 
distance at high speed. The quartz glass optical fiber is coupled to an 
optical transmit/receive module(two-way optical communication device) so 
as to be widely used for the two-way optical communication link such as a 
LAN that is based on an ATM(asynchronous transmission mode). 
However, the cost of the quartz glass optical fiber is high, the 
single-mode optical fiber needs to have a small diameter of merely several 
.mu.m in view of a problem on manufacturing, and furthermore, it is 
difficult and time-consuming to adjust a coupling to the optical 
transmit/receive module, resulting in an increase in cost. Consequently, 
it is difficult to adopt the quartz glass optical fiber for a small-scale 
network such as a home network. 
Meanwhile, examples of a multimode optical fiber are fibers including the 
quartz glass optical fiber and a plastic optical fiber(hereinafter, 
abbreviated as POF) whose core is made of plastic. Under the present 
circumstances, it is difficult for the POF to make a transmission over a 
long distance because of its relatively great transmission loss; however, 
the materials are inexpensive, the bending loss is small, resistance to 
cracking is offered, and a fiber with a large diameter of approximately 1 
mm can be easily manufactured. Therefore, the POF makes it easy to adjust 
the coupling to the optical transmit/receive module and to reduce the cost 
of installing; consequently, the POF is suitable for a small-scale network 
such as a home network. 
FIG. 22 illustrates an example of the two-way optical communication link 
which includes the POF serving as a medium. Here, two POFs 102 are 
respectively provided for transmission and reception. As a light-emitting 
element 107 on the transmitting side, an LED or a semiconductor laser is 
adopted and is coupled to the POF 102 directly or via a lens. on the 
receiving side, a photodiode is used as a light-receiving element 106 so 
as to receive light transmitted from the POF 102. 
Such a two-way optical communication link has an advantage of easily 
adjusting the light-receiving elements 106 and the light-emitting elements 
107 with the POFs 102 by making use of the large core diameters of the 
POFs 102. However, this arrangement requires two POFs 102, thereby 
increasing the cost in the case of transmission over a long distance. 
Further, Japanese Laid-Open Patent Publication No. 191543/1983 (Tokukaisho 
58-191543) discloses an optical transmit/receive module which is capable 
of two-way communication by using one optical fiber. As shown in FIG. 23, 
the optical transmit/receive module has a construction in which (a)a 
light-emitting element 207 which has a round light-emitting surface for 
launching light into an optical fiber 202 and (b) a ring-shaped 
light-receiving element 206 for receiving light incident from the optical 
fiber 202 are concentrically formed, an insulating space 210 being 
provided therebetween. 
With the above-mentioned arrangement, upon transmitting, light emitted from 
the light-emitting element 207 is directly transmitted to the optical 
fiber 202, and upon receiving, light incident from the optical fiber 202 
is received by the light-receiving element 206; therefore, it is possible 
to transmit and receive light merely with one optical fiber 202. 
However, the optical transmit/receive module which is provided with the 
light-emitting element 207 at the center of the light-receiving element 
206 causes the following problems: the light-emitting element 207 or the 
light-receiving element 206 is adversely affected due to heat of the 
light-emitting element 207, and for example, a stray light, which is 
transmitted from the light-emitting element 207 and is reflected on the 
incident surface of the optical fiber 202, may easily enter the 
light-receiving element 206, resulting in degradation in sensitivity to 
reception. 
Moreover, the above-mentioned optical transmit/receive module requires a 
face-emitting type of the light-emitting element 207 due to a structural 
constraint; however, in the case of a face-emitting type of the LED, it is 
difficult to increase the speed. Furthermore, with regard to the 
semiconductive laser which is capable of increasing the speed, the 
face-emitting type has not been put into practical use; therefore, this 
arrangement offers drawbacks with reliability and cost. 
Further, as shown in FIG. 24, a method in which a half mirror 310 
separately handles transmitted light and received light has been known. 
With this method, by changing the incident angle with the half mirror 310, 
light emitted from a light-emitting element 307 enters into an optical 
fiber 302, and light incident from the optical fiber 302 passes through 
the half mirror 310 and is received on a light-receiving element 306; 
therefore, it is possible to transmit and receive light merely with one 
optical fiber 302. 
However, in the method in which the half mirror 310 separately handles 
transmitted light and received light, with regard to the transmitted light 
and the received light, a loss of approximately 3dB occurs on the half 
mirror 310 and it is difficult to adjust an optical axis; consequently, 
this method tends to decrease the reliability on transmission and 
reception of light. 
In addition to the aforementioned methods, another method which transmits 
and receives light by using one optical fiber adopts an optical branch 
path of an optical waveguide. For the optical waveguide, materials such as 
glass, a semiconductor, and a plastic are now under study. Thanks to its 
small loss, a glass optical waveguide is used for the optical 
transmit/receive module with the single-mode optical fiber serving as the 
transmitting medium. Further, as disclosed in Japanese Laid-Open Patent 
Publication No. 188402/1991 (Tokukaihei 3-188402), a plastic optical 
waveguide can be easily worked and can be handled in a relatively simple 
way, thereby receiving attention as a substitute for the glass optical 
waveguide. 
However, it is difficult to work on a thick film of the glass optical 
waveguide; thus, in the case when the glass optical waveguide is coupled 
to the multimode optical fiber such as the POF with a large diameter, the 
coupling loss increases. Moreover, with regard to the plastic optical 
waveguide which is available for the optical transmit/receive module being 
appropriate for coupling to the multimode optical fiber, no publication 
has been disclosed yet. 
Meanwhile, Japanese Laid-Open Patent Publication No. 334644/1996 
(Tokukaihei 8-334644) discloses an optical branch device made of plastic. 
As shown in FIG. 25, the optical branch device is constituted by a clad 
410 which is composed of a single molded body made of resin, and a core 
403 which branches into not less than two paths in the clad 410. In the 
optical branch device, the end of the core 403 is coupled to the POF so 
that light incident from the POF propagates through the core 403 and 
branches off a branching portion 414 before having been launched. 
Such a plastic optical branch device makes it easy to form the core 403 
having the same size as the core diameter of the POF and provides a 
high-efficiency coupling to the multimode optical fiber such as the POF 
that has a large core diameter. 
However, in such a plastic optical branch device, the incident light is 
divided into virtually equal amounts at the branching portion 414 so that 
in the case when the plastic optical branch device is used as the optical 
transmit/receive module, the amount of received light is reduced in half. 
Therefore, this arrangement causes degradation in quality for reproducing 
a signal from the received light and a reduction in the reliability. In 
addition, the core 403 and the clad 410 are formed by using a molding 
operation; thus, it is difficult to achieve an integration with the 
light-receiving element and the light-emitting element, and to design a 
smaller version. 
SUMMARY OF THE INVENTION 
The present invention is devised in order to solve the above-mentioned 
problems. The objective is to provide a two-way optical communication 
device in which, unlike an optical branch device, incident light is 
transmitted to a light-receiving element with high efficiency, light 
emitted from a light-emitting element is transmitted to an optical fiber 
with high efficiency, a simple adjustment enables a coupling to a 
multimode optical fiber such as a POF having a relatively large core 
diameter with small loss, a two-way optical communication is achieved with 
one optical fiber, the transmission loss is small, the sensitivity to 
reception is high, the effect caused by stray light is small, and 
suitability for a small-scale network is exhibited; and to provide a 
two-way optical communication apparatus which uses the two-way optical 
communication device. 
Further, another objective of the present invention is to provide the 
two-way optical communication device in which a coupling is readily 
provided between a light-emitting element and an optical waveguide, an 
inexpensive and small version is achieved, an integration with other 
elements is readily possible, and suitability for a small-scale network is 
exhibited; and to provide a two-way optical communication apparatus which 
uses the two-way optical communication device. 
In order to achieve the above-mentioned objectives, a two-way optical 
communication device of the present invention, which carries out a two-way 
optical communication with an optical fiber, includes a main waveguide 
which is optically coupled to the optical fiber and directs light incident 
from the optical fiber to a light-receiving element, and a sub waveguide 
which is optically coupled to the side of the main waveguide with regard 
to an optical axis direction thereof and directs light emitted from the 
light-emitting element to the optical fiber. 
With the above-mentioned arrangement, the sub waveguide is coupled to the 
side of the main waveguide so as to have the optical axis which is 
different from that of the main waveguide, and light incident from the 
optical fiber cannot be readily coupled to the sub waveguide so as to be 
efficiently directed to the main waveguide and be coupled to the 
light-receiving element. 
Meanwhile, light emitted from the light-emitting element is directed to the 
sub waveguide so as to be coupled to the main waveguide and is efficiently 
coupled to the optical fiber from the main waveguide. (or the light is 
directly coupled to the optical fiber instead of the main waveguide.) 
Therefore, the aforementioned arrangement makes it possible to carry out a 
two-way optical communication with one optical fiber. 
Further, with the aforementioned arrangement, a conventional optical branch 
device is not necessary and branch loss of received light is small. 
Therefore, it is possible to improve quality of a signal reproduced from 
incident light and to consequently improve reliability of the optical 
communication. 
Furthermore, the light-emitting element and the light-receiving element are 
separately disposed on ends of different waveguides so that heat of the 
light-emitting element does not effect on the light-receiving element and 
it is possible to prevent a part of emitted light, which is reflected as 
stray light, from degrading sensitivity to reception. 
Moreover, a face-emitting type of the light-emitting element is not 
necessary so that it is possible to adopt a reliable and low-cost 
semiconductor laser as the light-emitting element. 
Additionally, another two-way optical communication device of the present 
invention, which carries out a two-way communication with an optical 
fiber, includes a main waveguide which is optically coupled to the optical 
fiber and directs light incident from the optical fiber to a 
light-receiving element, a first sub waveguide which is optically coupled 
to the side of the main waveguide with regard to the optical axis 
direction thereof and directs light emitted from a first light-emitting 
element to the optical fiber, and a second sub waveguide which is 
optically coupled to the side of the main waveguide with regard to the 
optical axis direction thereof and directs light emitted from a second 
light-emitting element to the optical fiber, the first and second 
light-emitting elements emitting light with different wavelengths. 
With the above-mentioned arrangement, it is possible to carry out a 
wavelength multiplex communication with one optical fiber by using a 
plurality of sub waveguides so that high-density information, namely, more 
pieces of information can be transmitted and received via the optical 
fiber. 
The two-way optical communication device of the present invention 
preferably emits light with a wavelength which is different from that of 
the two-way optical communication device disposed on the other end of the 
optical fiber. 
With the above-mentioned arrangement, the wavelengths of incident light and 
emitted light are different from each other so that it is easy to 
discriminate between incident light and stray light which is emitted light 
partly reflected so as to be directed from the sub waveguide to the main 
waveguide, a full-duplex two-way optical communication can be carried out 
with one optical fiber, and it is possible to increase the transmitting 
speed of the two-way optical communication. 
In order to achieve the above-mentioned objectives, a two-way optical 
communication apparatus of the present invention which carries out a 
two-way optical communication with an optical fiber, includes an optical 
fiber and a plurality of two-way optical communication devices having a 
main waveguide which is optically coupled to the optical fiber and directs 
light incident from the optical fiber to a light-receiving element, and a 
sub waveguide which is optically coupled to the side of the main waveguide 
with regard to the optical axis direction thereof and directs light 
incident from a light-emitting element to the optical fiber, the two-way 
optical communication devices being disposed on respective ends of the 
optical fiber. 
With the above-mentioned arrangement, the sub waveguide is coupled to the 
side of the main waveguide so as to have an optical axis which is 
different from that of the main waveguide, and light incident from the 
optical fiber cannot be readily coupled to the sub waveguide so as to be 
efficiently directed to the main waveguide and coupled to the 
light-receiving element. 
Meanwhile, light emitted from the light-emitting element is directed by the 
sub waveguide so as to be coupled to the main waveguide and is efficiently 
coupled to the optical fiber from the main waveguide. 
Therefore, the aforementioned arrangement makes it possible to efficiently 
carry out a two-way optical communication with one optical fiber. 
Further, with the aforementioned arrangement, a conventional optical branch 
path is not necessary, and branching loss of received light is small. 
Therefore, it is possible to improve quality of a signal reproduced from 
incident light and consequently to improve reliability. 
Furthermore, the light-emitting element and the light-receiving element are 
separately disposed on ends of different waveguides so that heat of the 
light-emitting element does not effect on the light-receiving element and 
it is possible to prevent a part of emitted light, which is reflected as 
stray light, from degrading sensitivity to reception. 
Moreover, a face-emitting type of the light-emitting element is not 
necessary so that it is possible to adopt a reliable and low-cost 
semiconductor laser as the light-emitting element. 
For a fuller understanding of the nature and advantages of the invention, 
reference should be made to the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS 
EMBODIMENT 1 
Referring to FIGS. 1 through 19, the following explanation describes a 
first embodiment of the present invention. 
FIG. 2 schematically shows the construction of a two-way optical 
communication link(two-way optical communicating apparatus) in accordance 
with the first embodiment. A two-way optical communicating link 16 is 
provided with(a) a cord-shaped optical fiber 2 for providing two-way 
transmission of light, which is modulated so as to be suitable for 
transmitting, in accordance with a data signal to be transmitted, and (b) 
optical transmit/receive modules (two-way optical communicating devices)1 
which is connected so as to be optically coupled to the respective ends of 
the optical fiber 2. 
The optical fiber 2 is provided with a core which is positioned at the 
center with an optically transmitting property and which has a round shape 
in cross section with regard to the direction orthogonally to an optical 
axis, and a clad which covers the core with virtually the even thickness 
and has an optically transmitting property. Therefore, the optical axis of 
the core virtually corresponds to the core of the optical fiber 2. 
The refractive index of the core is set larger than that of the clad. With 
regard to such an optical fiber 2, from the viewpoint of geometrical 
optics, the refractive index difference between the core and the clad is 
set so as to allow light traveling from the core toward the clad to be 
totally reflected. Thus, light is totally reflected on the boundary 
between the core and the clad. Therefore, light propagating through the 
core is enclosed in the core so as not to leak(diverge); consequently, 
this arrangement allows the optical fiber 2 to transmit light such as 
modulated light from one end to the other of the optical fiber 2 with 
small loss. 
As shown in FIG. 1, the optical transmit/receive module 1 is constituted by 
a light-emitting element 7 for generating the modulated light, a 
light-receiving element 6 for receiving the modulated light from the 
optical fiber 2 so as to generate a data signal, a main waveguide 3, which 
has a parallelepiped shape(rectangular shape) and an optically 
transmitting property, for propagating light incident from the optical 
fiber 2 to the light-receiving element 6, and a sub waveguide 4, which has 
a parallelepiped shape and an optically transmitting property, for 
transmitting light emitted from the light-emitting element 7 via the main 
waveguide 3 to the optical fiber 2. 
The light-emitting element 7, the light-receiving element 6, the main 
waveguide 3, and the sub waveguide 4 are formed so as to be integrated 
with a control device 10 for respectively controlling the light-receiving 
element 6 and the light-emitting element 7 and a monitoring photodiode 9 
for monitoring the output of the light-emitting element 7 on a substrate 5 
which is made of materials such as silicon by using the following 
semiconductor process. 
The end face along the length of the optical fiber 2 is closely opposed to 
a main-waveguide end face 11 positioned on the opposite side of the 
emitting side of the main waveguide 3 that is optically coupled to the 
light-receiving portion of the light-receiving element 6, so that the 
optical fiber 2 is optically coupled to the main waveguide 3. Therefore, 
the main waveguide 3 is arranged so that the direction of the length of 
the main waveguide 3 is virtually positioned along the optical axis 
direction of light which travels from the optical fiber 2 through the main 
waveguide 3. 
Further, the sub waveguide 4 is provided with the light-emitting element 7 
which is optically coupled to one end face along the length of the sub 
waveguide 4. The other end face of the sub waveguide 4 is coupled to the 
main waveguide 3 on the side surface(side portion) thereof with regard to 
the optical axis direction of the main waveguide 3 so as to emit light 
transmitted from the light-emitting element 7 toward the main-waveguide 
end face 11. Here, the optical axis direction is an optical axis direction 
of light travelling from the optical fiber 2 to the light-receiving 
element 6. The optical fiber 2 is coupled to the main waveguide 3. The sub 
waveguide 4 is coupled to the main waveguide 3 between the light-receiving 
element 6 and the main-waveguide end face 11 of the main waveguide 3, for 
example, at the midpoint therebetween. 
The light-emitting element 7 is controlled by the control device 10 so as 
to emit light to be transmitted. The light transmitted from the 
light-emitting element 7 is coupled to the sub waveguide 4 so as to 
propagate through the sub waveguide 4. And then, the light is coupled to 
the main waveguide 3 so as to propagate through the main waveguide 3. 
Furthermore, the transmitted light is coupled to the optical fiber 2 on 
the main-waveguide end face 11 and propagates through the optical fiber 2. 
A part of light emitted from the light-emitting element 7 is received by 
the monitoring photodiode 9 so as to be monitored. In accordance with the 
result of the monitoring, a feedback control adjusts the output of the 
light-emitting element 7 so as to keep even the intensity of light emitted 
from the light-emitting element 7. 
Meanwhile, received light incident from the optical fiber 2 is coupled to 
the main waveguide 3 on the main-waveguide end face 11 so as to propagate 
through the main waveguide 3. And then, the light is coupled to the 
light-receiving element 6 via an optical coupler 8 so as to be converted 
into an electric signal. Afterwards, the electric signal is decoded into a 
data signal in the control device 10. 
With the present embodiment, the sub waveguide 4 is coupled to a portion on 
the side surface of the main waveguide 3 so as to realize an optical 
two-way transmission and reception of light with one optical fiber 2. 
Therefore, the conventional optical branch path is omitted so that it is 
possible to achieve the high-efficiency optical transmit/receive module 1 
and two-way optical communication link 16 which are able to reduce branch 
loss of the received light. 
Moreover, a cross-sectional area with regard to the direction orthogonal to 
the optical axis of the main waveguide 3 is arranged so as to be larger 
than a cross-sectional area with regard to the direction orthogonal to the 
optical axis of the sub waveguide 4. Thus, when the received light travels 
from the optical fiber 2 to the main waveguide 3, it is possible to reduce 
the amount of light propagated to the sub waveguide 4 at a coupling 
portion 14 positioned between the main waveguide 3 and the sub waveguide 
4; consequently, light can be received with high efficiency. Furthermore, 
light transmitted from the light-emitting element 7 has modes the number 
of which is smaller than that of received light so that it is possible to 
propagate light with small loss despite of the small cross-sectional area 
of the sub waveguide 4. 
Additionally, it is desirable to set the number of modes of light 
propagating through the main waveguide 3 larger than that of light 
propagating through the sub waveguide 4. The optical fiber 2 which 
propagates multimode light contains a number of modes. Therefore, in the 
same manner, the number of modes of the main waveguide 3 increases so as 
to couple the optical fiber 2 and the main waveguide 3 with high 
efficiency. 
Meanwhile, it is desirable to arrange the sub waveguide 4 so as not to 
readily allow the entry of the received light and so as to efficiently 
propagate light transmitted from the light-emitting element 7. The 
light-emitting element 7 such as a semiconductor laser has a smaller 
number of modes; thus, it is readily possible to set the number of modes 
of the sub waveguide 4 smaller than that of the main waveguide 3. On the 
other hand, this arrangement regulates light which is directed to the main 
waveguide 3 and is directed to the sub waveguide 4, due to the large 
number of modes; therefore, the received light can efficiently propagate 
through the main waveguide 3 to the light-receiving element 6, and 
transmitted light can efficiently propagate to the main waveguide 3. 
It is possible to set an arbitrary number of modes in accordance with a 
cross-sectional area of each core layer(core portion) of the main 
waveguide 3 and the sub waveguide 4, and the refractive index difference 
between the core layer and the clad portion. Further, with regard to the 
optical fiber 2, for example, even when light with a low mode(small number 
of modes) such as semiconductor laser enters, the light varies the number 
of modes in accordance with the property of the optical fiber 2 while 
propagating through the optical fiber 2. 
With the multimode optical fiber 2(especially, a plastic optical fiber) 
which is used for the present invention, light propagating through the 
optical fiber 2 contains a large number of modes. Therefore, although the 
number of modes is small before transmission, light transmitted from the 
optical transmit/receive module 1 increases the number of modes while 
propagating through the optical fiber 2. 
In the present invention, for example, the width of the sub waveguide 4 is 
set at approximately 50 .mu.m; meanwhile, the core diameter of the optical 
fiber 2 is set at approximately 1 mm; thus, with regard to light which is 
incident from the optical fiber 2 and propagates through the main 
waveguide 3, the number of modes is larger than that of light propagating 
from the sub waveguide 4 to the main waveguide 3. 
As the optical fiber 2, the multimode optical fiber such as a POF is 
adopted. The core of the POF is made of plastic such as 
PMMA(PolyMethylMethAcrylate) and polycarbonate that is superior in an 
optically transmitting property. The clad is made of plastic whose 
refractive index is lower than that of the core. 
This optical fiber 2 allows the core diameter to be set at between 
approximately 200 .mu.m and 1 mm so that it is possible to set the 
diameter larger as compared with the case of a quartz optical fiber which 
is a single-mode optical fiber. The POF whose core is made of PMMA 
exhibits the highest transmitting rate at a wavelength in the vicinity of 
650 nm, and the POF whose core is made of polycarbonate exhibits the 
highest transmitting rate at a wavelength in the vicinity of 780 nm. 
As compared with the quartz optical fiber, the POF has a large transmission 
loss; however, thanks to the small bending loss, resistance to bending, 
and ability to easily manufacture a fiber with a large diameter, it is 
easy to adjust the coupling with the optical transmit/receive module 1, 
and the two-way optical communication link 16 can be obtained at low cost. 
As the light-emitting element 7, it is possible to adopt, for example, 
semiconductor laser for emitting coherent light that is made of a material 
selected from GaAlAs and GsInAlP, or a light-emitting diode(LED). Since it 
is difficult to communicate at high speed with the LED, the LED is 
available at the speed of not more than approximately 150 Mbps. Meanwhile, 
the semiconductor laser makes it possible to communicate by using a high 
band for a transmitting rate exceeding 150 Mbps. The semiconductor laser 
for generating wavelengths of 650 nm and 780 nm, that allows a high 
transmissivity of the POF core, is used for a digital versatile disk or 
CD(compact disk) and is highly reliable at low cost. 
Therefore, a core material of the optical fiber 2 is decided in accordance 
with the wavelength used for the light-emitting element 7. In the case of 
the semiconductor laser with the wavelength of 650 nm, the POF whose core 
is made of PMMA is adopted. In the case of the semiconductor laser with 
the wavelength of 780 nm, the POF whose core is made of polycarbonate is 
adopted. Therefore, it is possible to achieve the highly reliable two-way 
optical communication link 16 which reduces transmission loss at low cost. 
Furthermore, HPCF is also acceptable as the optical fiber 2. The HPCF 
includes a core made of quartz glass and a clad made of a hard polymer. 
The HPCF is therefore more expensive than the POF; however, the 
transmission loss is small and the transmission band is wide. The HPCF is 
used as a transmitting medium so as to obtain the two-way optical 
communication link for communicating over a long distance at higher speed. 
In this case, in view of transmissivity for each wavelength of the HPCF, 
it is desirable to adopt the light-emitting element 7 which has the 
wavelength of 780 nm or 850 nm. 
As a light-receiving element 6, a photodiode is used, which converts the 
intensity of the incident modulated light into an electric signal and has 
high sensitivity within the wavelength range of the light-emitting element 
7. For example, it is preferable to adopt photodiodes including a PIN 
photodiode made of materials such as silicon and an avalanche photodiode. 
Furthermore, the light-receiving element 6 can be formed so as to be 
embedded in the substrate 5. 
The main waveguide 3 and the light-receiving element 6 are coupled to each 
other via the optical coupler 8. As shown in FIGS. 3 and 4, the optical 
coupler 8, for example, provides an optical coupling between the 
light-emitting element 6 embedded in the substrate 5 and the main 
waveguide 3 formed on the light-receiving surface. On the side of the main 
waveguide 3 that faces the substrate 5, a buffer layer 3b is formed as the 
clad portion in order to prevent light from leaking to the substrate 5. 
The buffer layer 3b is made of, for example, materials such as silicon 
oxide whose refractive index is lower than that of a core layer 3a. 
Here, as shown in FIG. 4, on the light-receiving surface of the 
light-receiving element 6, the optical coupler 8 has a construction in 
which the buffer layer 3b is formed in a tapered shape so as to have a 
thickness which gradually becomes smaller along the traveling direction of 
received light, and the core layer 3a is formed so as to have a thickness 
which gradually becomes larger in order to make a complement to the 
thinner part. In the optical coupler 8, received light, which propagates 
through the main waveguide 3, leaks to the substrate 5 from the thinner 
part of the buffer layer 3b or the part on which no buffer layer 3b exists 
so that the received light is coupled to the light-receiving element 6. 
Here, for example, a reflection protection film made of materials such as 
silicon nitride is also allowed to be formed between the main waveguide 3 
and the light-receiving element 6. 
Such an optical coupler 8 makes it possible to efficiently couple light 
which has propagated through the main waveguide 3 to the light-receiving 
element 6. Further, the light-receiving element 6 is covered with the main 
waveguide 3 so that stray light cannot readily enter the light-receiving 
element 6. Furthermore, the light-receiving element 6 is embedded in the 
substrate 5, and the main waveguide 3 is formed thereon; thus, the 
light-receiving element 6 and the main waveguide 3 can be formed in 
accordance with a semiconductor manufacturing process, and it is possible 
to readily adjust the positions of the light-receiving element 6 and the 
main waveguide 3 with high precision. Moreover, it is possible to realize 
a smaller version of the optical transmit/receive module 1. 
FIG. 5 shows another construction of the optical fiber 8, in which the 
buffer layer 3b is not formed into a tapered shape but is cut on the 
light-receiving element 6; namely, a part of the buffer layer 3b that has 
been cut is entirely replaced by the core layer 3a.The end face of the 
buffer 3b which has been cut is arranged so as to be orthogonal to the 
optical axis direction of the main waveguide 3. 
Such a working can be easily performed on the buffer layer 3b in the 
direction orthogonal to the surface of the substrate 5 by using methods 
such as a reactive ion etching(RIE). This makes it possible to simplify 
the manufacturing process of the optical coupler 8. Moreover, the 
light-receiving element 6 is allowed to be coupled to the main waveguide 3 
by using another method in which the optical fiber 8 is not used. For 
example, the light-receiving element 6 can be bonded to the end face of 
the main waveguide 3 that is arranged in the direction of the optical 
axis. 
Further, as shown in FIG. 6, a reflection surface 15 is formed on the end 
face portion of the main waveguide 3(end face portion arranged in the 
optical axis direction of the main waveguide 3) so that light propagating 
through the main waveguide 3 is reflected on the reflection surface 15 to 
the light-receiving element 6 so as to be coupled to the main waveguide 3. 
When the light-receiving element 6 is coupled by using the methods shown 
in FIGS. 4 and 5, in some cases, light is not totally transmitted to the 
substrate 5 and a part of the light leaks by being emitted from the end 
face portion of the main waveguide 3. For this reason, as shown in FIG. 6, 
the reflection surface 15 forces light to be coupled to the 
light-receiving element 6; consequently, it is possible to improve the 
coupling efficiency. 
As shown in FIG. 1, as the aforementioned monitoring photodiode 9, it is 
possible to adopt, for example, a PIN photodiode, etc. made of silicon. 
The monitoring photodiode 9 monitors light emitted from the back side of 
the light-emitting element 7(surface opposite to the surface for emitting 
light to the sub waveguide 4) so as to maintain the output of the 
light-emitting element 7 at a certain level. The monitoring photodiode 9 
is also allowed to be embedded in the substrate 5. Further, the 
light-emitting element 7 and the monitoring photodiode 9 can be coupled to 
each other via a monitoring optical waveguide. The monitoring optical 
waveguide is used for coupling so that it is possible to reduce stray 
light occurring from the light-emitting element 7. 
The control device 10 decodes an electric signal of light received on the 
light-receiving element 6 into a data signal, controls the output of the 
light-emitting element 7, and adjusts the output of the light-emitting 
element 7 in accordance with light for monitoring that is received on the 
monitoring photodiode 9. 
All of the main waveguide 3, the sub waveguide 4, the light-receiving 
element 6, the light-emitting element 7 are allowed to be formed on the 
substrate 5. All these members are formed on the same substrate 5 so that 
it is possible to realize a smaller version of the optical 
transmit/receive module 1 and to manufacture them by one operation. 
Manufacturing by one operation makes it possible to improve the precision 
and the reliability. For the substrate 5, it is possible to adopt 
semiconductors made of materials including silicon and gallium arsenide, 
glass, and resin. 
The following explanation describes the main waveguide 3 and the sub 
waveguide 4 in detail. 
As shown in FIG. 7, the main waveguide 3 is, for example, formed into a 
rectangular shape on the substrate 5 and is provided with the core layer 
3a and the buffer layer 3b which is formed between the core layer 3a and 
the substrate 5. The refractive index of the core layer 3a is smaller than 
that of the core layer 3a. Further, on the sides of the core layer 3a 
arranged in the direction of the optical axis, except for the side on 
which the buffer layer 3b is formed, an over clad layer 3c is allowed to 
be formed. The over clad layer 3c has a "]" shaped cross-sectional area 
with regard to the direction orthogonal to the optical axis and has a 
refractive index which is smaller than that of the core layer 3a. The sub 
waveguide 4 has the same construction as the main waveguide 3; thus, the 
explanation thereof is omitted. 
Furthermore, as shown in FIG. 8, the main waveguide 3 is, for example, 
formed by using a method such as a molding method instead of being formed 
on the substrate 5. In this case, the cross-sectional area of the core 
layer 3a of the main waveguide 3 is formed into a round shape so that it 
is possible to improve the efficiency of coupling to the optical fiber 2. 
Such a main waveguide 3 is provided with the over clad layer 3c on the rim 
of the core layer 3a. The sub waveguide 4 provided for the main waveguide 
3 has the same construction as the main waveguide 3; thus, the explanation 
thereof is omitted. The core layer 3a is also allowed to be made of 
materials such as polyimide, PMMA, polycarbonate, and polystyrene, or to 
be made of plastics by using these materials as main components. 
As compared with the optical waveguide made of quartz, the optical 
waveguide made of plastics makes it easy to form a thicker film. And it is 
possible to efficiently couple the optical waveguide made of plastics to 
the multimode optical fiber which has a large diameter. Furthermore, the 
optical waveguide made of plastics can be readily manufactured at low 
cost. 
In the case when a semiconductive laser is used as the light-emitting 
element 7, during a bonding operation for the substrate 5, it is necessary 
to perform a heating operation at a temperature of approximately 
300.degree. C.; however, plastics generally cause a problem due to its low 
temperature resistance. Meanwhile, polyimide has a high heat resistance of 
not less than approximately 300.degree. C. as compared with other plastics 
so that it is difficult for a heating operation to alter the quality. 
Furthermore, polyimide fluoride has a high transparency so as to reduce 
the transmission loss. For this reason, it is preferable to adopt 
polyimide for the core layer 3a. Specifically, products such as PIX or 
OPI(manufactured by Hitachi Chemical Co., Ltd.) can be adopted as 
polyimide. 
For the buffer layer 3b and the over clad layer 3c, materials whose 
refractive indexes are smaller than that of the core layer 3a are adopted. 
For instance, materials such as silicon oxide and plastics are available. 
Moreover, for the over clad layer 3c, photoresist or thermosetting resin 
can be adopted. 
Next, referring to FIG. 9, the following explanation describes the 
manufacturing method of the main waveguide 3 and the sub waveguide 4. The 
main waveguide 3 and the sub waveguide 4 can be manufactured by using the 
semiconductor process. An example of the manufacturing process is 
described as follows: 
1) On the substrate 5 made of silicon, as the buffer layer 3b, silicon 
oxide is made into a film with the thickness of several .mu.m by using a 
sputtering method. And then, photoresist is applied thereon, and a 
patterning operation is performed thereon by using photolithography. With 
photoresist serving as a mask, unnecessary portions of the buffer layer 3b 
are removed by the reactive ion etching(RIE) in which CF.sub.4 gas is 
used(shown in FIG. 9(a)). The surface of the substrate 5 is previously 
subjected to a rubbing and polishing operation. On the substrate 5, the 
aforementioned light-receiving element and monitoring photodiode, and the 
wiring thereof are previously formed(not shown). 
2) As the core layer 3a, polyimide(product name "PIX3400" manufactured by 
Hitachi Chemical Co., Ltd.) is applied on the substrate 5 by using a spin 
coat. Afterwards, baking operations are performed at temperatures of 
130.degree. C., 230.degree. C., and 350.degree. C. The film thickness of 
the core layer 3a is arranged so as to be approximately 40 .mu.m after the 
baking operations(shown in FIG. 9(b)). 
3) On the core layer 3a, a silicon oxide film 12, which serves as a mask of 
the core layer 3a, is formed with a thickness of approximately 1 by using 
the spattering method. Further, on the silicon oxide film 12, photoresist 
13 is applied so as to serve as a mask of the silicon oxide film 12(shown 
in FIG. 9(c)). 
4) The photoresist 13 is patterned into the shapes of the main waveguide 3 
and the sub waveguide 4 by using photolithography, and unnecessary 
portions of the silicon oxide film 12 are removed in accordance with the 
shapes of the main waveguide 3 and the sub waveguide 4 by using the RIE 
with CF.sub.4 gas(shown in FIG. 9(d)). 
5) And then, by using the RIE with oxygen gas, unnecessary portions of the 
core layer 3a are removed in accordance with the shapes of the main 
waveguide 3 and the sub waveguide 4. At this time, the photoresist 13 is 
also removed(shown in FIG. 9(e)). The silicon oxide film 12 cannot be 
removed but used as an upper clad. As a mask of the core layer 3a, in 
addition to the silicon oxide film 12, materials such as resist containing 
silicon and aluminum can be used; however, these materials tend to cause 
residues(remaining portions) due to the RIE using oxygen gas. 
As described above, in the manufacturing process, the main waveguide 3 and 
the sub waveguide 4 are manufactured in accordance with the semiconductor 
process; thus, it is possible to realize a high-precision manufacturing 
process and coupling to the light-receiving element 6. Further, a 
plurality of optical transmit/receive modules 1 are manufactured on the 
substrate 5 by one operation so that it is possible to realize a low 
manufacturing cost. The aforementioned manufacturing method of the main 
waveguide 3 and the sub waveguide 4 is one of the examples; naturally, the 
method can be partially changed and another method is also available for 
manufacturing. For example, the main waveguide 3 and the sub waveguide 4 
can be formed by using the molding operation. 
Referring to FIG. 10, the following explanation describes the shapes of the 
main waveguide 3 and the sub waveguide 4. As illustrated in FIG. 10, the 
optical axis of the main waveguide 3 is virtually in parallel with the 
optical axis of the optical fiber 2. The optical axis of the sub waveguide 
4 is arranged so as to be inclined by .theta.a with regard to the optical 
axis of the main waveguide 3. The sub waveguide 4 is coupled to the main 
waveguide 3 at a distance L from the main-waveguide end face 11 of the 
main waveguide 3. The optical axis of the main waveguide 3 and the optical 
axis of the optical fiber 2 are arranged so as to be virtually in parallel 
with each other so that it is possible to efficiently couple light, which 
is incident from the optical fiber 2 to the main waveguide 3 and to reduce 
the transmission loss after the light has entered the main waveguide 3. 
Meanwhile, the optical axis of the sub waveguide 4 is inclined with regard 
to the optical axis of the main waveguide 3 so that light which is 
incident from the optical fiber 2 and propagates through the main 
waveguide 3 cannot be readily coupled to the sub waveguide 4. Especially, 
the angle .theta.a is arranged so as to be formed by the optical axis of 
the main waveguide 3 and the optical axis of the sub waveguide 4 so that 
it is possible to obtain the optical transmit/receive module 1 which 
causes smaller loss upon transmission and reception. 
FIG. 11 shows a result of calculation on a relationship between the angle 
.theta.a and loss appearing after light emitted from the light-emitting 
element 7 has propagated through the sub waveguide 4 and has been coupled 
to the main waveguide 3. The calculation is performed by using a beam 
propagating method. Here, it is assumed that two kinds of semiconductor 
lasers are used as the light-emitting element 7: a semiconductor laser 
with a wavelength of 650 nm and a semiconductor laser with a wavelength of 
780 nm. It is also assumed that the property of polyimide(product name 
"PIX" manufactured by Hitachi Chemical Co., Ltd.) is used for the main 
waveguide 3 and the sub waveguide 4, and silicon oxide is used for the 
buffer layer 3b and the over clad layer 3c. The width of the main 
waveguide 3 is 400 .mu.m and the width of the sub waveguide 4 is 100 
.mu.m. The above-mentioned widths are arranged orthogonally to the optical 
axes of the main waveguide 3 and the sub waveguide 4 and in parallel with 
the surface of the substrate 5. 
Light which is emitted from the light-emitting element 7 is coupled to the 
sub waveguide 4, propagates through the sub waveguide 4, and is coupled to 
the main waveguide 3. The transmission loss differs in accordance with a 
variation of the angle .theta.a. As shown in FIG. 11, with regard to any 
one of the wavelengths, the transmission loss sharply increases when the 
angle .theta.a exceeds 22.degree.. For this reason, it is desirable to set 
the angle .theta.a at not more than 22.degree.. Further, the lengths of 
the main waveguide 3 and the sub waveguide 4 are arranged in such a state 
that the light-receiving element 6 and the light-emitting element 7 do not 
interfere with each other. Therefore, the larger the angle .theta.a, it is 
possible to further reduce the lengths of the main waveguide 3 and the sub 
waveguide 4. 
Meanwhile, light incident from the optical fiber 2 propagates through the 
main waveguide 3, and a part of the light is distributed to the sub 
waveguide 4 at the coupling portion 14. In order to increase the amount of 
light received by the light-receiving element 6, it is necessary to reduce 
the amount of light distributed to the sub waveguide 4. FIG. 12 shows a 
result of a calculation on a relationship between the angle .theta.a and 
branch loss appearing upon propagating light from the main waveguide 3 to 
the sub waveguide 4. The calculation is performed by using the beam 
propagating method. Here, it is assumed that the conditions of the main 
waveguide 3 and the sub waveguide 4 are the same as those of calculation 
of FIG. 11. The smaller the angle .theta.a, the loss appearing at the 
coupling portion 14 tends to increase. 
However, within a 10.degree. to 20.degree. range of the angle .theta.a, the 
loss is not more than 1 dB. As shown in FIG. 10, the main waveguide 3 is 
arranged in line with the optical fiber 2, and the sub waveguide 4 is 
coupled at somewhere along the main waveguide 3 so that it is possible to 
obtain the optical transmit/receive module 1 which causes smaller 
propagation loss upon transmission and reception. 
The smaller the distance L between the main-waveguide end face 11 and the 
coupling portion 14, the shorter the length of the main waveguide 3 can be 
arranged; consequently, it is possible to realize a smaller version of the 
optical transmit/receive module 1. Further, the propagation 
loss(transmission loss) can be reduced in the main waveguide 3. As shown 
in FIG. 13, in the case when the distance L is short, for example, if the 
distance L is set at 0, transmitted light which propagates through the sub 
waveguide 4 is coupled to the optical fiber 2 in virtually a direct 
manner. 
In general, the main waveguide 3 tends to cause large loss on the sides due 
to a problem of a manufacturing process. Therefore, in such a construction 
in which transmitted light is directly coupled to the optical fiber 2, it 
is possible to reduce the loss. In this case, the angle .theta.b is formed 
by the optical axis of the sub waveguide 4 and the optical axis of the 
optical fiber 2 so as to further reduce the loss. 
FIG. 14 shows a result of a calculation on a relationship between the angle 
.theta.b and loss appearing in the optical fiber 2 after light has entered 
the optical fiber 2. The calculation is performed by using the beam 
propagating method. Two kinds of combinations of the light-emitting 
element 7 and the optical fiber 2 are assumed: a semiconductor laser with 
a wavelength of 650 nm and a POF with a PMMA core, and a semiconductor 
laser with a wavelength of 780 nm and a POF with a polycarbonate core. 
Here, the core diameter of the optical fiber 2 is set at 500 .mu.m, the 
width of the main waveguide 3 is set at 400 .mu.m, and the width of the 
sub waveguide 4 is set at 100 .mu.m. 
Light which is emitted from the light-emitting element 7 is coupled to the 
sub waveguide 4, propagates through the sub waveguide 4, and is coupled to 
the optical fiber 2. At this time, the loss differs in accordance with a 
variation of the angle .theta.b. As shown in FIG. 14, with regard to any 
combination, the loss sharply increases when the angle .theta.b exceeds 
170. For this reason, it is desirable to set the angle .theta.b at not 
more than 170. Further, the lengths of the main waveguide 3 and the sub 
waveguide 4 are arranged in such a state that the light-receiving element 
6 and the light-emitting element 7 do not interfere with each other. 
Therefore, the larger the angle .theta.b, it is possible to further reduce 
the lengths of the main waveguide 3 and the sub waveguide 4. 
Both of the main waveguide 3 and the sub waveguide 4 are formed in a 
straight line. These are formed in a straight line so as to reduce the 
transmission loss, and upon manufacturing, for example, upon manufacturing 
by using the semiconductor process, the main waveguide 3 and the sub 
waveguide 4 can be readily formed. 
In addition, FIG. 15 shows one example of a variation in which the optical 
axis of the main waveguide 3 is inclined by an angle .theta.c with regard 
to the optical axis of the optical fiber 2. In this case, in order to 
reduce the loss of the transmitted light, the angle .theta.c is set at not 
more than 17.degree. in accordance with the same reason as that described 
referring to FIG. 14. 
As described above, the optical axis of the main waveguide 3 is inclined 
with regard to the optical axis of the optical fiber 2 so that it is 
possible to set larger the cross-sectional area of the main-waveguide end 
face 11, and an efficient coupling to the optical fiber 2 can be realized. 
The width and thickness of the sub waveguide 4 is determined in accordance 
with the efficiency of coupling to the light-emitting element 7. With 
regard to the coupling efficiency of light emitted from the light-emitting 
element 7 and the sub waveguide 4, FIGS. 16 and 17 respectively show 
calculation results of the width and thickness dependence of the sub 
waveguide 4. Here, the thickness is measured orthogonally to the optical 
axes of the main waveguide 3 and the sub waveguide 4 and orthogonally to 
the surface of the substrate 5. 
The calculation is performed by using a ray tracing method. Further, the 
calculation is performed on the assumption that semiconductor laser is 
used as the light-emitting element 7, an intensity distribution is a gauss 
distribution, a radiation angle (half band width) with regard to the 
direction orthogonal to the surface of the substrate 5 is 30.degree., and 
a radiation angle with regard to the direction parallel to the surface of 
the substrate 5 is 10.degree.. Here, the distance between the 
light-emitting element 7 and the sub waveguide 4 is 30 .mu.m, and the 
install accuracy of the light-emitting element 7 is .+-.15 .mu.m both in 
the directions of the width and thickness of the sub waveguide 4. 
Incidentally, in the case of coupling to a single-mode optical fiber, the 
accuracy needs to be not more than several .mu.m; however, as described 
above, the accuracy within the range of .+-.15 .mu.m allows the 
light-emitting element 7 to be readily installed. 
The calculation is performed on the assumption that the axis is shifted to 
a maximum degree. If it is assumed that the tolerance of the coupling 
efficiency is 1 dB, referring to FIGS. 16 and 17, it is understood that 
the thickness needs to be not less than 35 .mu.m and the width needs to be 
not less than 40 .mu.m with regard to the sub waveguide 4. Namely, the 
thickness is set at not less than 35 .mu.m and the width is set at not 
less than 40 .mu.m with regard to the sub waveguide 4 so that it is 
possible to narrow a range of variation of loss even in the event of the 
shift of the axis of the light-emitting element 7. It is desirable that 
the thickness of the main waveguide 3 be arranged so as to be the same as 
that of the sub waveguide 4 in order to simplify the manufacturing 
process. 
The following explanation describes the coupling between the main waveguide 
3 and the optical fiber 2. With regard to the optical fiber 2 with a large 
diameter for transmitting multimode light, emitted light is not regarded 
as a point source; thus, it is difficult to gather light with a lens. 
Therefore, it is desirable to provide the coupling in a state in which the 
optical fiber 2 and the main waveguide 3 directly face each other; namely, 
each end face of them closely opposes to each other. FIG. 18 shows a 
calculation result on the relationship between the coupling loss and the 
distance between the optical fiber 2 and the main-waveguide end face 11. 
The total amount of loss is determined by adding (a) the coupling loss of 
received light travelling from the optical fiber 2 to the main waveguide 3 
and (b)the coupling loss of the transmitted light travelling from the main 
waveguide 3 to the optical fiber 2. The calculation is performed on the 
assumption that the core diameter of the optical fiber 2 is the same as 
the width of the main waveguide 3(500 .mu.m). Here, the main waveguide 3 
has a thickness of 80 .mu.m, and the optical fiber 2 has two kinds of an 
aperture number(referred to as NA in the figure), 0.3 and 0.5. As 
described in FIG. 18, the shorter the distance between the optical fiber 2 
and the main-waveguide end face 11, the coupling efficiency improves. The 
distance is favorably set at not more than 100 .mu.m. It is more desirable 
to set the distance at not more than 50 .mu.m so as to prevent a reduction 
in the coupling efficiency. 
The main-waveguide end face 11 is also allowed to have a protection film 
such as a cover glass on the surface thereof in order to avoid damage 
resulting from a contact with the optical fiber 2. Further, it is allowed 
to fill a gap of the coupling portion between the main-waveguide end face 
11 and the optical fiber 2 with a refractive index adjusting agent, which 
has a refractive index close to the refractive indexes of the 
main-waveguide end face 11 and the optical fiber 2. 
FIG. 19 shows a calculation result on the relationship between (a)the 
coupling efficiency of the main waveguide 3 and the optical fiber 2 and 
(b)the width of the main waveguide 3. The loss is determined by adding 
(a)the coupling loss of received light travelling from the optical fiber 2 
to the main waveguide 3 and (b)the coupling loss of the transmitted light 
travelling from the main waveguide 3 to the optical fiber 2. It is assumed 
that the optical fiber 2 has two kinds of an aperture number, 0.3 and 0.5, 
the thickness of the main waveguide 3 is 80 .mu.m, and the distance 
between the optical fiber 2 and the main-waveguide end face 11 is 50 
.mu.m. 
As described in FIG. 19, when the width of the main waveguide 3 is less 
than 0.8 times the core diameter of the optical fiber 2, the coupling 
efficiency sharply declines. For this reason, it is desirable to set the 
width of the main waveguide 3 at not less than 0.8 times the core diameter 
of the optical fiber 2. 
Naturally, since the above-mentioned arrangement is one of the examples, 
the construction is not particularly limited to this example. For 
instance, the main waveguide 3 and the sub waveguide 4 can be respectively 
arranged in a curve, or the width or the thickness can vary in a tapered 
shape. 
As described above, with the optical transmit/receive module 1 in 
accordance with the first embodiment, a simple adjustment achieves a 
low-loss coupling to the multimode optical fiber, such as the POF, whose 
core diameter is relatively large, merely one optical fiber 2 enables a 
two-way communication, the transmission loss is small upon transmitting 
and receiving, influence of stray light is reduced, the coupling to the 
optical fiber 2 can be readily provided via the light-emitting element 7 
and the sub waveguide 4, integration with other elements can be readily 
achieved, and a smaller version can be realized at low cost. 
EMBODIMENT 2 
Next, referring to FIG. 20, the following explanation describes the second 
embodiment. However, in the second embodiment, those members that have the 
same functions and that are described in the first embodiment are 
indicated by the same reference numerals and the description thereof is 
omitted. 
In the second embodiment, a plurality of sub waveguides, for example, a 
first sub waveguide 4a and a second sub waveguide 4b are formed on a main 
waveguide 3. The first sub waveguide 4a and the second sub waveguide 4b 
are respectively coupled to the main waveguide 3 on the sides thereof. 
Light emitted from a first light-emitting element 7a, which is coupled to 
the first sub waveguide 4a, propagates through the first sub waveguide 4a. 
The light is coupled to the main waveguide 3 and enters the optical fiber 
2. Light emitted from a second light-emitting element 7b, which is coupled 
to the second sub waveguide 4b, propagates through the second sub 
waveguide 4b. The light is coupled to the main waveguide 3 and enters the 
optical fiber 2. 
The first light-emitting element 7a and the second light-emitting element 
7b are arranged so as to emit light with different wavelengths such as 780 
nm and 650 nm. Further, between the main waveguide 3 and a light-receiving 
element 6, for example, a wave-length separating element such as an 
interference filter can be provided so as to allow merely light having the 
wavelength of the received light to enter. Moreover, the number of sub 
waveguides 4 can be not less than three. 
With this arrangement of the optical transmit/receive module 1, it is 
possible to achieve a wave-length multiplex communication by using a 
plurality of the sub waveguides 4. As described above, a two-way optical 
communication link described in the second embodiment enables the 
wave-length multiplex communication, and it is possible to transmit and 
receive high-density information, namely, a larger amount of information 
via the optical communication. 
EMBODIMENT 3 
Next, referring to FIG. 21, the following explanation describes the third 
embodiment. However, in the third embodiment, those members that have the 
same functions and that are described in the first and second embodiments 
are indicated by the same reference numerals and the description thereof 
is omitted. 
With the arrangement of the embodiment 3, a light-emitting element 7 of an 
optical transmit/receive module 1 is arranged so as to emit light with a 
wavelength of .lambda.1, which is different from a wavelength .lambda.2 of 
a light-emitting element of a second optical transmit/receive module(not 
shown) disposed on the other end of an optical fiber 2. And, on an optical 
path(optical axis) between a main waveguide 3 and a light-receiving 
element 6, a wavelength separating element 17 such as an interference 
filter is provided. The wavelength separating element 17 transmits light 
having the wavelength .lambda.2 but shield light having the wavelength 
.lambda.1. 
Transmitted light which is emitted from the lightemitting element 7 
propagates through a sub waveguide 4 and enters the optical fiber 2. 
However, a part of the transmitted light is reflected on a main-waveguide 
end face 11 or the end face of the optical fiber 2. Hence, in the case 
when the wavelengths .lambda.1 and .lambda.2 are identical with each 
other, it is not possible to discriminate between the reflected light and 
light simultaneously transmitted from the second optical transmit/receive 
module(not shown). Therefore, in this case, merely a half-duplex 
communication can be carried out in a stable manner. 
However, with the arrangement of the third embodiment, the wavelengths 
.lambda.1 and .lambda.2 are arranged so as to be different from each 
other, and the wave-length separating element 17 separates light in 
accordance with each wavelength; thus, it is possible to carry out a 
full-duplex communication in a stable manner. 
Naturally, the second optical transmit/receive module which is coupled to 
the other end of the optical fiber 2 is arranged so as to have the 
transmitted light with wavelength .lambda.2 and the received light with 
wavelength .lambda.1. Further, with regard to a method for separating 
wavelengths, it is possible to adopt light-receiving elements 6 whose 
wavelength sensitivities are different from each other, in addition to a 
method using the wavelength separating element 17. The wavelength 
separating element 17 is provided between the main waveguide 3 and the 
light-receiving element 6 so that it is possible to separate the 
wavelengths in a more positive manner. 
As described above, a two-way optical communication link described in the 
third embodiment achieves the full-duplex communication with one optical 
fiber 2 and improves the transmitting speed of the two-way optical 
communication. 
As described above, a first two-way optical communication device is 
characterized by a main waveguide which is optically coupled to an optical 
fiber and directs light incident from the optical fiber to a 
light-receiving element, and a sub waveguide which is optically coupled to 
the side of the main waveguide with regard to the direction of an optical 
axis of the main waveguide and directs light incident from a 
light-emitting element to the optical fiber via the main waveguide. 
In accordance with the first two-way optical communication device, the sub 
waveguide is coupled to the side of the main waveguide so that the optical 
axis of the sub waveguide is formed in a different direction from the 
optical axis of the main waveguide. Thus, received light which is directed 
from the optical fiber to the main waveguide cannot be readily coupled to 
the sub waveguide; consequently, the received light can be coupled to the 
light-receiving element with high efficiency. 
Further, with the first two-way optical communication device, light 
transmitted from the light-emitting element is coupled to the main 
waveguide from the sub waveguide, and the light is coupled to the optical 
fiber from the main waveguide with high efficiency. For this reason, the 
first two-way optical communication device achieves the two-way optical 
communication with one optical fiber. 
Furthermore, with the first two-way optical communication device, in the 
case when an optical fiber for propagating multimode light is adopted as 
an optical fiber, the core diameter of the optical fiber can be arranged 
larger than that of a single-mode optical fiber so that it is possible to 
optically couple between the main waveguide and the optical fiber, which 
has a large core diameter, in a simpler manner as compared with the case 
of the conventional single-mode optical fiber. 
Moreover, with the first two-way optical communication device, even if a 
conventional optical branch device is omitted, it is possible to achieve 
the two-way optical communication for handling transmitted light and 
received light so that transmission loss and influence of stray light are 
reduced; therefore, it is possible to increase sensitivity to reception 
with a simple and inexpensive construction. 
In addition, with the first two-way optical communication device, it is 
possible to efficiently provide a two-way optical communication with one 
optical fiber at low cost; therefore, this arrangement is preferably 
adopted for a two-way optical communication apparatus(two-way optical 
communication link) for a small-scale network. 
As described above, with the arrangement of the first two-way optical 
communication device, the second two-way optical communication device of 
the present invention is characterized in that the main waveguide is 
arranged so as to set a number of modes of light propagating through the 
main waveguide larger than that of light propagating through the sub 
waveguide. 
With the second two-way optical communication device, the number of modes 
of light propagating through the main waveguide is set larger than that of 
light propagating through the sub waveguide so that light received from 
the optical fiber cannot readily propagate through the sub waveguide; 
thus, received light can propagate to the light-receiving element with 
high efficiency. Further, transmitted light can also propagate through the 
sub waveguide with high efficiency. Consequently, it is possible to 
achieve high-efficiency transmission and reception as compared with the 
case of the conventional optical branch path. 
As described above, with the arrangement of the first two-way optical 
communication device, the third two-way optical communication device of 
the present invention is characterized in that a cross-sectional area with 
regard to the direction orthogonal to the optical axis of the main 
waveguide is set larger than that of the sub waveguide. 
With the third two-way optical communication device, the cross-sectional 
area of the main waveguide is set larger than that of the sub waveguide so 
that light incident from the multimode optical fiber can be efficiently 
coupled to the main waveguide. Meanwhile, the sub waveguide cannot be 
readily coupled to light incident from the optical fiber so that it is 
possible to couple light emitted from the light-emitting element to the 
optical fiber with high efficiency. 
As described above, with the arrangement of the first, second, or third 
two-way optical communication device, the fourth two-way optical 
communication device of the present invention is characterized in that the 
core portions of the main waveguide and the sub waveguide are made of 
plastic. 
With the fourth two-way optical communication device, the core portions of 
the main waveguide and the sub waveguide are made of plastic so that it is 
easy to work on a thick film and it is possible to efficiently provide a 
coupling to the optical fiber with a large diameter by making a simple 
adjustment. 
As described above, with the arrangement of the fourth two-way optical 
communication device, the fifth two-way optical communication device of 
the present invention is characterized in that the core portions are made 
of plastic which is mainly composed of polyimide. 
With the fifth two-way optical communication device, the core portions of 
the main waveguide and the sub waveguide are made of plastic which is 
mainly composed of polyimide so that it is possible to realize the main 
waveguide and the sub waveguide which have high heat resistance and high 
optical transmissivity; thus, even when semiconductor laser is used as the 
light-emitting element, it is possible to realize the main waveguide and 
the sub waveguide which can reduce adverse effect of a heating operation 
with small loss, upon forming an ohmic electrode in installing the 
semiconductor laser. 
Further, it is possible to work by using a dry etching technology so that 
manufacturing is possible by using a semiconductor process. For example, 
it is possible to readily work on the substrate made of semiconductor, and 
highly-precision manufacturing is available at low cost. 
As described above, with the arrangement of any one of the first through 
fifth two-way optical communication devices, the sixth two-way optical 
communication device of the present invention is characterized in that an 
angle formed by the optical axes of the sub waveguide and the main 
waveguide is set at not more than 22.degree.. 
With the sixth two-way optical communication device, the angle formed by 
the optical axes of the sub waveguide and the main waveguide is set at not 
more than 22.degree.; thus, it is possible to couple light, which is 
emitted from the light-emitting element and propagates through the sub 
waveguide, to the main waveguide with small loss. Further, it is possible 
to efficiently couple light, which is emitted from the optical fiber to 
the main waveguide, to the light-receiving element. 
As described above, with the arrangement of any one of the first through 
sixth two-way optical communication devices, the seventh two-way optical 
communication device of the present invention is characterized in that an 
angle, which is formed by the optical axis of the sub waveguide and the 
optical axis of the core of the optical fiber, is set at not more than 17. 
With the seventh two-way optical communication device, the angle, which is 
formed by the optical axis of the sub waveguide and the optical axis of 
the core of the optical fiber, is set at not more than 17.degree. so that 
light which is emitted from the main waveguide to the optical fiber is 
totally reflected within the optical fiber with high efficiency before 
propagating; therefore, this arrangement cannot readily cause loss within 
the optical fiber and it is possible to efficiently transmit light through 
the optical fiber. 
As described above, with the arrangement of any one of the first through 
seventh two-way optical communication devices, the eighth two-way optical 
communication device of the present invention is characterized in that an 
angle, which is formed by the optical axis of the main waveguide and the 
optical axis of the core of the optical fiber, is set at not more than 
17.degree.. 
With the eighth two-way optical communication device, the angle, which is 
formed by the optical axis of the main waveguide and the optical axis of 
the core of the optical fiber, is set at not more than 17.degree.; 
therefore, light incident from the main waveguide to the optical fiber 
cannot be readily damaged within the optical fiber so that light can be 
efficiently transmitted, and it is possible to set the large 
cross-sectional area of the main waveguide at the coupling portion to the 
optical fiber so that light received from the optical fiber can be 
efficiently transmitted to the light-receiving element. 
As described above, with the arrangement of any one of the first through 
eighth two-way optical communication devices, the ninth two-way optical 
communication device of the present invention is characterized in that the 
main waveguide is provided with a plurality of sub waveguides. 
With the ninth two-way optical communication device, a plurality of the sub 
waveguides are provided; thus, a plurality of the light-emitting elements 
have different wavelengths for each of the sub waveguides so as to readily 
perform a wavelength multiplex communication; consequently, the two-way 
optical communication can be achieved with high density. 
As described above, with the arrangement of any one of the first through 
ninth two-way optical communication devices, the tenth two-way optical 
communication device of the present invention is characterized in that the 
main waveguide and the sub waveguide are formed on a substrate. 
With the tenth two-way optical communication device, the main waveguide and 
the sub waveguide are formed on the same substrate; therefore, integration 
with other elements can be readily carried out so as to achieve a smaller 
version of the tenth two-way optical communication device. Furthermore, it 
is possible to simplify the manufacturing process. 
As described above, with the arrangement of the tenth two-way optical 
communication device, the eleventh two-way optical communication device of 
the present invention is characterized in that an optical coupler is 
provided for inducing light, which is incident from the optical fiber to 
the main waveguide, to the light-receiving element formed on the 
substrate. 
With the eleventh two-way optical communication device, the light-receiving 
element can be covered with the main waveguide so that stray light cannot 
readily enter the light-receiving element. Further, the light-receiving 
element is formed on the substrate and the main waveguide is formed 
thereon so that a manufacturing process can be carried out by using the 
semiconductor process; therefore, it is possible to readily adjust the 
positions of the light-receiving element and the main waveguide with high 
precision. 
Moreover, the eleventh two-way optical communication device enables a 
manufacturing process using the semiconductor process so that it is 
possible to achieve a smaller version of the eleventh two-way optical 
communication device. Furthermore, the eleventh two-way optical 
communication device is capable of coupling between the main waveguide and 
the sub waveguide by using an optical coupler and is capable of coupling 
light propagating the main waveguide to the light-receiving element with 
high efficiency. 
Consequently, with the eleventh two-way optical communication device, the 
optical coupler, which directs received light from main waveguide to the 
light-receiving element, is provided on the main waveguide which covers 
the light-receiving element formed on the substrate so that it is possible 
to achieve a smaller version and improve the sensitivity to reception at 
low prices. 
As described above, with the arrangement of the tenth or eleventh two-way 
optical communication device, the twelfth two-way optical communication 
device of the present invention is characterized in that the width of the 
sub waveguide is set at not less than 40 .mu.m at least at the portion on 
which the sub waveguide is coupled to the light-emitting element. 
With the twelfth two-way optical communication device, the width of the sub 
waveguide is set at not less than 40 .mu.m at the portion on which the sub 
waveguide is coupled to the light-emitting element so that in the case 
when, for example, semiconductor laser is used as the light-emitting 
element, it is possible to couple between the sub waveguide and the 
semiconductor laser with a simple adjustment. 
As described above, with the arrangement of any one of the tenth through 
twelfth two-way optical communication devices, the thirteenth two-way 
optical communication device of the present invention is characterized in 
that the width of the sub waveguide is set at not less than 35 .mu.m at 
least at the portion on which the sub waveguide is coupled to the 
light-emitting element. 
With the thirteenth two-way optical communication device, the width of the 
sub waveguide is set at not less than 35 .mu.m at the portion on which the 
sub waveguide is coupled to the light-emitting element so that in the case 
when, for example, semiconductor laser is used as the light-emitting 
element, it is possible to couple between the sub waveguide and the 
semiconductor laser with a simple adjustment. 
As described above, with the arrangement of any one of the tenth through 
thirteenth two-way optical communication devices, the fourteenth two-way 
optical communication device of the present invention is characterized in 
that the thickness of the main waveguide is the same as that of the sub 
waveguide. 
With the fourteenth two-way optical communication device, the thickness of 
the main waveguide is the same as that of the sub waveguide; therefore, 
the main waveguide and the sub waveguide are simultaneously formed in 
accordance with the semiconductor manufacturing process so that it is 
possible to readily form both of the main waveguide and the sub waveguide. 
As described above, with the arrangement of any one of the tenth through 
fourteenth two-way optical communication devices, the fifteenth two-way 
optical communication device of the present invention is characterized in 
that the width of the main waveguide is set at not less than 0.8 times as 
large as the core diameter of the optical fiber at least at the portion on 
which the main waveguide is coupled to the optical fiber. 
With the fifteenth two-way optical communication device, the width of the 
main waveguide is set at not less than 0.8 times as large as the core 
diameter of the optical fiber at least at the portion on which the main 
waveguide is coupled to the optical fiber so that it is possible to couple 
light incident from the optical fiber to the main waveguide with high 
efficiency and to couple light incident from the main waveguide to the 
optical fiber with high efficiency. 
As described above, with the arrangement of any one of the tenth through 
fifteenth two-way optical communication devices, the sixteenth two-way 
optical communication device of the present invention is characterized in 
that the light-emitting element coupled to one end of the optical fiber 
has a light-emitting wavelength which is different from that of the 
light-emitting element coupled to the other end of the optical fiber. 
With the sixteenth two-way optical communication device, the light-emitting 
element coupled to one end of the optical fiber has a light-emitting 
wavelength which is different from that of the light-emitting element 
coupled to the other end of the optical fiber so that two-way full-duplex 
communication is achieved with one optical fiber. 
As described above, with the arrangement of the sixteenth two-way optical 
communication device, the seventeenth two-way optical communication device 
of the present invention is characterized in that a wavelength separating 
element is provided between the main waveguide and the light-receiving 
element. 
With the seventeenth two-way optical communication device, the wavelength 
separating element separates wavelengths so as to control light entering 
the light-emitting element, thereby regulating entry of light having 
unnecessary wavelength. Thus, it is possible to carry out a full-duplex 
communication in a more positive manner. 
As described above, a first two-way optical communication apparatus which 
carries out a two-way optical communication with a plurality of the 
two-way optical communication devices, each being optically coupled to 
each end of the optical fiber for propagating multimode light, is 
characterized in that at least one of a plurality of the two-way optical 
communication devices is any one of the first through seventeenth two-way 
optical communication devices. 
With the first two-way optical communication apparatus, even if a 
conventional optical branch device is omitted, the two-way optical 
communication for handling transmitted light and received light can be 
achieved so as to reduce transmission loss and influence of stray light; 
therefore, it is possible to increase sensitivity to reception with a 
simple construction. 
As described above, with the arrangement of the first two-way optical 
communication apparatus, the second two-way optical communication 
apparatus is characterized in that a plastic optical fiber whose core is 
made of plastic is adopted as an optical fiber. 
With the second two-way optical communication apparatus, a POF is used as 
an optical fiber so that bending loss is small, resistance to cracking is 
offered, and a fiber with a large diameter of approximately 1 mm can be 
easily manufactured; therefore, the POF makes it easy to adjust the 
coupling between the optical fiber and the main waveguide and to lower the 
price of the two-way optical communication device. 
As described above, with the arrangement of the second two-way optical 
communication apparatus, the third two-way optical communication apparatus 
is characterized in that in the case when the core is made of plastic 
mainly composed of a polymethyl methacrylate resin, the light-emitting 
element emits light with a wavelength of approximately 650 nm. 
With the second two-way optical communication apparatus, a plastic whose 
core is mainly composed of polymethyl methacrylate is adopted as a POF, 
and a semiconductor laser or a LED with a wavelength of approximately 650 
nm is adopted as the light-emitting element so that it is possible to 
realize the lower-priced two-way optical communication device which is 
highly reliable with small loss. 
As described above, with the arrangement of the second two-way optical 
communication apparatus, the fourth two-way optical communication 
apparatus is characterized in that in the case when the core is made of 
plastic which is mainly composed of a polycarbonate resin, the 
light-emitting element emits light with a wavelength of approximately 780 
nm. 
With the fourth two-way optical communication apparatus, a plastic whose 
core is mainly composed of polycarbonate is adopted as the POF, and a 
semiconductor laser or a LED with a wavelength of approximately 780 nm is 
adopted as the light-emitting element so that it is possible to realize 
the lower-priced two-way optical communication device which is highly 
reliable with small loss. 
As described above, with the arrangement of the first two-way optical 
communication apparatus, the fifth two-way optical communication device is 
characterized in that a hard-polymer clad fiber, in which the clad is made 
of plastic and the core is made of quartz, can be adopted as the optical 
fiber. 
With the fifth two-way optical communication apparatus, a hard-clad quartz 
optical fiber, in which the clad is made of a hard polymer and the core is 
made of quartz, is adopted as the optical fiber serving as a medium so 
that the wide transmitting band makes it possible to communicate at high 
speed over a long distance. 
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.