Optical transmission device

A fiber optic cable has a light-exiting face. A cable-adapter is affixed to the light-exiting face. The cable-adapter has a cylindrical outer surface, a diameter of which is gradually tapered down. Light leaving the face of the cable is moved towards an optical receptor and gradually converged, by virtue of the tapered cylindrical surface, of the cable-adapter and emitted. In this manner, an optical loss is minimized. To prevent a leak through the tapered surface, the tapered surface is coated with a low refractive index film. Further, a ferrule is used to position the cable-adapter relative to the fiber optic cable. By virtue of this configuration, an optical leakage can be minimized when the light is received by an optical receptor having a small surface chip.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light communication system. In 
particular, the present invention is directed to a device that enables a 
transmission between an optical fiber and a light receptor, and, 
specifically, an optical transmission device that is used to transmit a 
signal between an optical fiber and a light receptor. 
2. Description of Background Information 
Generally, two main types of optical fiber cables are employed; glass 
optical fibers and plastic optical fibers. 
Glass optical fibers have a higher transparency, as compared to plastic 
optical fibers. When the precision of the light transmission is to be 
improved, the diameter of the core part of the optical fiber is designed 
to be as small as possible. As a result, the ability of the glass optical 
fiber to carry a single mode signal is enhanced. 
On the other hand, plastic optical fibers offer a better resistance to 
bending and countermeasures to an optical interference or attenuation in 
the fiber can be envisaged. The diameter of a plastic optical fiber is 
usually greater than the diameter of a glass optical fiber. Due to the 
larger diameter, plastic optical fibers tend to suffer from a certain 
level of mode dispersion, thereby causing light transmission loss and wave 
deformation. Nonetheless, due to the relatively large diameter of the 
optical fiber, high precision alignment of one optical fiber to another 
optical fiber (or an optical fiber to an element, such as, for example, a 
receptor) is not required. Plastic optical fibers are therefore easily 
optically aligned, are less costly than glass optical fibers, and thus, 
are used in optical communications devices. Because of its large diameter, 
a known plastic fiber can receive a certain number of optical signals 
emitted by a light emitting diode (LED) into this optical fiber. These 
optical signals may then exit and be received by a large chip-size 
photodiode (PD), which transforms the optical signals into electrical 
signals. 
In recent optical fiber-communications, research has taken place into high 
speed optical communications having a speed of, for example, several 
hundreds of Mbps (megabits per second) to an order of several Gbps 
(gigabits per second). A surface of a receptor-side photodiode, to be 
irradiated by a light to generate excited electrons, has a limited chip 
size. The electrons move to a position located at a chip terminal, where 
wires are bonded. The greater the chip surface of the photodiode, the 
longer it takes for the electrons to move from one point to another point 
(e.g., electron movement time increases). Therefore, the chip surface of 
the photodiode should be designed to be as small as possible. 
However, in the case of a photodiode having a small surface chip, the known 
plastic optical fibers have been designed to have a large opening 
diameter, as mentioned above. As a result, as shown in FIG. 1 of the 
drawings, only part of the signal coming out of light-exit end face A2 of 
the fiber optic (optical fiber) A1 is received by the photodiode A3. Thus, 
the optical loss is increased between the light-exit end face A2 and the 
photodiode A3. 
When the plastic optical fiber has, for example, an end external diameter 
of 750 .mu.m, a uniform light can be emitted therefrom and received by a 
photodiode having a light-reception diameter face of 250 .mu.m. If 
calculated on the basis of the surface ratio, only 11% of the original 
light volume is received. This means that, when transformed into decibel 
milliwatt (dBm) ratings, the light volume is decreased by 9.54 dBm. 
Thus, a large open diameter plastic fiber A1, though easily handled, is 
poorly adapted to a photodiode A.sub.3 having a small chip surface 
designed for receiving a high speed signal, as a large loss occurs due to 
the bonding of a large diameter optical fiber to a small diameter 
photodiode. 
Accordingly, a purpose of the present invention is to provide an 
optical-transmission device which, when used with an optical receptor 
having a small surface chip, reduces the optical loss due to bonding. 
SUMMARY OF THE INVENTION 
In order to solve the above-mentioned problem, there is provided an 
optical-transmission device having an optical axis, comprising: 
a fiber optic cable (optical fiber cable) having a light-exit face of a 
predetermined diameter; 
cable-adapting means (cable-adapter) for connecting the fiber optic cable 
to a light-receiving element. The cable-adapting means (cable-adapter) 
comprises a frusto-conical shape having a tapered cylindrical surface with 
a large face and a small end face. The large face has a diameter 
substantially equal to that of the light-exit face of the fiber optic 
cable, the diameter gradually decreasing (tapering) as it goes away from 
the light-exit face along an optical axis; and 
a ferrule that covers at least the light-exit face of the fiber optic cable 
and the large face of the cable-adapting means. 
Accordingly, light entering one end portion of the fiber optic cable is led 
through the fiber optic cable by reflecting on the tapered surface of the 
fiber optic cable. Therefore, when a small surface chip is used as an 
outside optical element, both units are bonded, such that the light is 
efficiently guided therebetween. The optical loss is thus greatly reduced. 
Further, the fiber optic cable and the cable-adapting means are properly 
positioned and fixed in the ferrule. These units can be positioned without 
using another element. The number of pieces used to couple the fiber optic 
cable to the photo receptor is thus reduced. 
The ferrule may comprise an inside surface corresponding to a tapered 
cylindrical surface of the cable-adapting means, thereby securely holding 
the cable-adapting means. Therefore, the cable-adapting means can be 
positioned more securely and easily in the ferrule. 
Moreover, the cable-adapting means may comprise an optical fiber (fiber 
optic) material having a same level of numerical aperture as that of the 
fiber optic cable. The numerical aperture of the fiber optic cable can 
therefore be matched with the numerical aperture of the cable-adapting 
means. Thus, light leaving the fiber optic cable can be efficiently led 
through the cable-adapting means. 
Advantageously, the fiber optic cable and the cable-adapting means, 
respectively, comprise a core portion having a same level of refractive 
index to the other. The fiber optic cable is bonded to the cable-adapting 
means through a transparent binding material having a same level of 
refractive index as that of the core portion. A Fresnel reflection is thus 
efficiently suppressed at the bonding interface, and the optical 
transmission of the cable is improved. 
According to another advantage of the present invention, the fiber optic 
cable and the cable-adapting means may be integrally formed. As a result, 
Fresnel reflection is prevented and an optical transmission is improved. 
Advantageously, the cable-adapting means comprise a clad portion having a 
same level of refractive index to that of the core portion. 
The clad portion may be covered with a coating having a refractive index 
that is lower than that of the clad portion. By virtue of this structure, 
light led to the tapered surface of the cable-adapting means and going 
therethrough for leaking is reflected back towards the interior of the 
cable-adapting means by the coating. As a result, an optical loss in the 
cable-adapting means is greatly reduced. 
According to another advantage of the present invention, the fiber optic 
cable may comprise a plastic optical-fibercable. The optical-fibercable 
may comprise, for example, a step-index type optical-fiber cable, or a 
graded-index type optical-fiber cable. 
According to an object of the present invention, an optical-transmission 
device having an optical axis is disclosed, comprising: 
a fiber optic cable having a light-exit face with a predetermined diameter; 
a cable-adapter that connects the fiber optic cable to a light-receiving 
element, the cable-adapter having a frusto-conical shape with a tapered 
cylindrical surface with a large face and a small end face, the large face 
having a diameter substantially equal to the predetermined diameter of the 
light-exit face of the fiber optic cable, the tapered cylindrical surface 
gradually decreasing as one moves from the large face towards the small 
end face along the optical axis; and 
a ferrule that covers at least the light-exit face of the fiber optic cable 
and the large face of the cable-adapter. 
According to an advantage of the instant invention, the ferrule has an 
inside surface corresponding to the tapered cylindrical surface of the 
cable-adapter, to thereby securely hold the cable-adapter. 
According to a feature of the present invention, the cable-adapter 
comprises an optical fiber material having a same level of numerical 
aperture as that of the fiber optic cable. 
According to another feature of the instant invention, the fiber optic 
cable and the cable-adapter respectively comprise a core portion having a 
same level of refractive index as the other, and are bonded to each other 
via a transparent binding material having a same level of refractive index 
as that of the core portion. 
A feature of the instant invention is that the fiber optic cable and the 
cable-adapter are integrally formed. Additionally, the fiber optic cable 
may be a plastic fiber optic cable, comprising either a step-index type 
fiber optic cable, or a graded-index type fiber optic cable. 
According to another advantage of the instant invention, the cable-adapter 
comprise a core portion and a clad portion. The clad portion has a same 
level of refractive index as that of the core portion. In addition, the 
clad portion may be covered with a coating having a refractive index lower 
than that of the clad portion. 
According to another object of the present invention, an 
optical-transmission device having an optical axis is disclosed, 
comprising: 
a fiber optic cable having a light-exit face with a predetermined diameter; 
cable-adapting means for adapting the fiber optic cable to a 
light-receiving element, the cable-adapting means having a certain shape 
with a tapered cylindrical surface and a large face and a small end face, 
the large face having a diameter substantially equal to the predetermined 
diameter of the light-exit face of the fiber optic cable, the tapered 
cylindrical surface gradually decreasing as one moves from the large face 
towards the small end face along the optical axis; and 
a ferrule that covers at least the light-exit face of the fiber optic cable 
and the large face of the cable-adapting means. 
The present disclosure relates to subject matter contained in Japanese 
Patent Application No. HEI 9-042423, filed on Feb. 26, 1997, which is 
expressly incorporated herein by reference in its entirety.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 illustrates an optical-transmission device according to a first 
embodiment of the present invention. As shown in FIG. 2, cable-adapting 
device (cable adapter) 2 has a substantially frusto-conical shape. The 
cable adapter 2 is bound to a light-exit end-face 1a of a plastic optical 
cable 1, such that a light signal inputted to the fiber optic cable at a 
light-receiving face 1b (by, for example, a light emitting diode (LED)), 
exits via a light-exit end-face 1a, and is injected onto a large face 2a 
of the cable adapter 2. The light signal then exits via a small face 2b of 
the cable adapter 2 to impinge upon a photodiode 31. In the disclosed 
embodiment, the cable-adapter 2 is housed into a ferrule 3. 
Fiber optic cable 1 typically has a relatively small difference of 
refractive index between its core portion and its clad portion. The fiber 
optic cable 1 illustrated in FIG. 1 has a low level of numerical aperture 
(NA), and belongs to a step-index type fiber. As shown in FIG. 3, the 
fiber optic cable 1 has a core portion 11 that is made of a plastic 
material having a refractive index of approximately 1.495, such as, for 
example, PMMA (polymethylmethacrylate) or other similar material. The 
optical-fiber cable 1 further includes a clad portion 12 that coats 
(covers) core portion 11. The clad portion 12 is made of a material, such 
as, for example, a fluorocarbon polymer having a refractive index of 
approximately 1.493, such as, for example, PTFE (polytetrafluoroethylene) 
or other similar material. The external cylindrical surface of the clad 
portion 12 is also covered with a coating 13 made of, for example, a 
plastic material, such as, for example, polyethylene or other plastic. The 
optical fiber cable 1 is stripped of the coating 13 from its end portion 
along a predetermined length, thereby exposing the light-exit end-face 1a. 
In the above embodiment, the optical-fibercable 1 has a clad external 
diameter of 750 .mu.m and gives a transmission loss of 230 dB/km. A 
difference of refractive index between the core portion 11 and the clad 
portion 12 is equal to approximately 0.002. 
As shown in FIG. 4, the cable-adapter (cable adapter device) 2 has a 
frusto-conical shape comprises a core 21 and a clad portion 22. A tapered 
surface of the cable adapter 2 is further covered by a low refractive 
index film or coating 23 (see FIG. 3). 
In the illustrated embodiment, cable-adapter 2 extends the length of the 
plastic optical fiber of the fiber optic cable 1, and has a numerical 
aperture that is the same as the optical fiber. For instance, 
cable-adapter 2 may have a length L1 of 6 mm. The frusto-conical shape, 
consisting of a core 21 and a clad portion 22, has a large face 2a having 
a large diameter Da measuring approximately 750 .mu.m, corresponding to 
the outer diameter of the optical fiber cable 1. The cable-adapter 2 also 
has a small face 2b having a diameter Dc that ranges from approximately 
200 .mu.m to 700 .mu.m in diameter. The core 21 and the clad portion 22 of 
the cable-adapter 2 has the same layer-thickness ratio as the core 11 and 
the clad portion 12 of the optical fiber cable 1. The same thickness ratio 
must be kept at the small face 2b of the cable-adapter 2, i.e., 
EQU Dc=.alpha.Db 
wherein Dc signifies the diameter of the core 21 at the small face 2b, Db 
signifies the diameter of the clad 22 at the small face 2b, and .alpha. 
signifies the layer thickness ratio (i.e., core diameter/clad portion 
diameter). 
The above-mentioned frusto-conical shape (21, 22) is formed by, for 
example, stacking the core 21 and the clad portion 22 on the optical fiber 
cable 1 and applying a force of approximately 100 g at the end portion 
thereof, in an axial direction while heating, thereby elongating the 
frusto-conical shape. FIG. 5 illustrates this method. A weight 30a of 
approximately 100 g is tied to the end portion of a plastic optical fiber 
A1 through a thread 30b. A portion adjacent to the end portion is heated 
to approximately 150.degree. C. by a stream of hot air passing along a 
span of approximately 10 mm of the plastic optical fiber. By varying the 
heating time from 5 to 10 seconds, the optical fiber A1 is expanded by 
virtue of the weight 30a, to obtain a cable-adapter 2 having a 
frusto-conical shape, as shown in FIG. 4. 
When an optical wave path is formed in the frusto-conical shape, the 
advancing light is reflected at the interface of the core 21 and the clad 
portion 22, as shown in FIG. 6. At every reflection, angles .psi..sub.1, 
.psi..sub.2, .psi..sub.3, . . . , .psi..sub.n of the advancing light, with 
respect to the optical axis, become greater. When .theta. is defined as an 
inclination of the tapered surface relative to the optical axis and 
.psi..sub.0 is defined as an angle of incidence of the light towards the 
core 21, the successive angles .psi..sub.1, .psi..sub.2, .psi..sub.3, . . 
. , .psi..sub.n of the advancing light vis-a-vis the optical axis can be 
formulated as follows: 
##EQU1## 
Subsequently, the angle .psi..sub.n after n-times reflections at the 
interface of the core 21 and the clad 22 can be defined as follows: 
EQU .psi..sub.n =.psi..sub.0 +2.theta..times.n 
This equation indicates that the reflection angle of the light at the 
interface of the core 21 and the clad portion 22 increases with the number 
of reflections. 
Accordingly, by repeating the reflections, the advancing light inside the 
fiber tends to go through the interface, which leads to increased light 
leakage. To prevent the leakage of light, the tapered surface of the 
cable-adapter 2 is painted with a low refractive index film 23, made of, 
for example, PMMA (polymethyl-methacrylate)or other similar material, 
which has a refractive index lower than that of the clad portion 22. By 
virtue of this structure, the light passing through the interface of the 
core 21 and the clad portion 22 is fed back into the core 21. In this 
construction, the small-diameter face 2b of the cable-adapter 2 has a 
numerical aperture (NA) of 0.5. 
The cable-adapter 2 is affixed (adhered) to the light-exit end-face 1a of 
the optical-fiber cable 1 through a transparent soft silicone elastomer 24 
(see FIGS. 2 and 3). This elastomer 24 is produced by extending a base 
material of a high refractive index silicone, forming a sheet that is 
hardened at room temperature, and carefully punching out the elastomer 24 
from the sheet. Such a silicone elastomer 24 is designed to have a 
refractive index of approximately 1.495, the figure being the same as that 
of the cores 11, 21 of the optical-fiber cable 1 and the cable-adapter 2, 
respectively. In this manner, a Fresnel reflection at the interface 
between the core 11, 21 and the soft silicone elastomer 24 is prevented. 
Alternatively, a matching oil could be used instead of the silicone 
elastomer 24 to attach (affix) the fiber optic cable 1 to the 
cable-adapter 2. However, the viscosity of the oil tends to vary with 
temperature and generates air bubbles when pouring. This may affect the 
stability of the communication path. Accordingly, the use of a 
silicone-based, high refraction material is preferably employed. 
The ferrule 3 may be manufactured, for example, by cutting a brass material 
in the shape illustrated in FIG. 7. The ferrule 3 is provided with a 
flange 25 that projects outward in a radial direction, half-way along an 
axial direction. The flange 25 abuts a connector (not shown in the 
drawings). The ferrule 3 further includes a cable path-hole 29 comprising 
a fiber housing 26 having a void diameter corresponding to the diameter of 
the coating 13 of the cable 1, a cable-adapter housing 27 that holds the 
cable-adapter 2 and has an exposed end portion and a slant housing 28 
connecting these two housings. The three housings are sequentially formed 
in the same axial direction. Moreover, the cable-adapter housing 27 has a 
void frusto-conical shape corresponding to the shape of the cable-adapter 
2. 
The functions of the above-mentioned device will be explained below. As 
shown in FIG. 2, the light-exit end-face 1a of the fiber optic cable 1 is 
adhered (stuck) to the cable-adapter 2 via the soft silicone elastomer 
sheet 24. The resulting assembly is inserted through the cable path hole 
29 of the ferrule 3. The photodiode (PD) 31 (shown in FIG. 2) is then 
placed proximate (e.g., face-to-face near) the end portion of the 
cable-adapter 2. 
In the disclosed embodiment, the photodiode 31 comprises a small surface 
chip that is used for high speed communication. A light source 32, such 
as, for example, a light emitting diode (LED) is positioned proximate 
(e.g., face-to-face near) the light-receiving face 1b of the fiber optic 
cable 1. 
The light emitting diode 32 is turned ON and OFF to emit a predetermined 
optical signal on the light-receiving face 1b. The light moves through the 
cable 1 by repeated reflections at the interface of the core 11 and the 
clad portion 12, and exits through the light-exit end-face 1a. The light 
passes through the soft silicone elastomer 24, which has the same 
refractive index as that of the cable, and goes through the large-diameter 
face 2a of the cable-adapter 2 into the core 21. Since the soft silicone 
elastomer 24 has the same level of refractive index as that of both cores 
11 and 21, the optical loss due to Fresnel reflection is efficiently 
minimized. 
The light penetrating into the core 21 of the cable-adapter 2 moves through 
the cable-adapter 2 by successive reflections at the interface of the core 
21 and the clad portion 22. During this procedure, as discussed above, the 
angle of reflection of the light at the interface of the core 21 and the 
clad portion 22 increases according to the number of reflections However, 
as the clad portion is further coated (painted) with a low refractive 
index film, light leakage is efficiently prevented. 
The light passes through the small diameter face 2a and impinges (hits) a 
receiving surface of the photodiode 31, which transforms the light into an 
electrical signal. The electrical signal is treated by a processing 
circuit 33 (see FIG. 2). As discussed above, the photodiode 31 comprises a 
small surface chip that operates at a high speed. When the small diameter 
face 2b of the cable-adapter 2 is designed to have a dimension 
corresponding to the chip surface of the photodiode 31, most of the light 
leaving the small diameter face 2b can be recovered by the light-receiving 
face of the photodiode 31, such that an optical loss incurred at this 
stage can be greatly reduced. 
Table 1 shows variations of the light-reception sensitivity of a 
photodiode, when the external diameter of the small face 2b of the 
cable-adapter 2 is varied. However, the photodiode used for these 
experiments has a large surface chip. Consequently, in the case of the 
maximal external diameter of 750 .mu.m, the exit-light from the 
cable-adapter 2 is considered to be fully (entirely) recovered by the 
photodiode. In these experiments, the fiber optic cable 1 has a length of 
approximately 2 m. The light-emitting diode 32 used in these experiments 
has a median wavelength of 700 nm and an output power of approximately 3 
dBm. The cable-adapter 2 have a length L1 of approximately 6 mm. The 
frusto-conical shape (consisting of core 21 and clad 22) comprises the 
large face 2a having a diameter Da of 750 .mu.m, which corresponds to the 
external diameter of the fiber optic cable 1. Clad portion 12 has an 
external diameter of 750 .mu.m. As such, when the small-diameterface 2b 
has an outer diameter of 750 .mu.m, tapering is totally removed. 
TABLE 1 
______________________________________ 
Small face of the cable-adapter 
(external diameter) Light reception sensitivity 
______________________________________ 
250 .mu.m -15.519 dBm 
300 .mu.m -14.974 dBm 
350 .mu.m -14.477 dBm 
400 .mu.m -14.821 dBm 
450 .mu.m -14.309 dBm 
500 .mu.m -13.719 dBm 
550 .mu.m -13.104 dBm 
600 .mu.m -12.413 dBm 
650 .mu.m -12.252 dBm 
700 .mu.m -11.722 dBm 
750 .mu.m -11.116 dBm 
(no tapered surface) 
______________________________________ 
In the above-mentioned experiments, the photodiode used has a large surface 
chip. In practice, when a photodiode 31 for high speed processing (small 
surface chip) has a receiving-surface diameter of, for instance, 250 
.mu.m, a final light-reception sensitivity must be calculated taking into 
account this difference in surface size. When light signals uniformly 
leave the end surface of the plastic fiber optic (having an external 
diameter of 750 .mu.m) and are captured by a photodiode having a diameter 
of 250 .mu.m, only 11% of the original signals are received on the basis 
of the surface ratio. This can be transformed into dBm as follows: 
EQU -10.times.Log(0.11)=9.54(dB); 
indicating that the optical signal level (volume) is decreased (lowered) by 
9.54 dBm. 
On the other hand, Table 1 shows that when the small diameter face of 750 
.mu.m is compared with 250 .mu.m, the light-reception sensitivity is 
lowered by only approximately 4.4 dBm (-11.116 dBm minus -15.519 dBm). 
By adding the cable-adapter 2 to the fiber optic cable 1, the 
light-reception sensitivity is lowered by about 4.4 dBm (-15.519). In 
comparison, the past (prior art) devices exhibit a sensitivity loss 
(decrease) of 9.54 dBm, calculated on the basis of the surface ratio, with 
respect to the original light intensity (-11.116). Thus, compared with the 
prior art example, a gain of 
EQU 9.54-4.4=5.14dBm 
is obtained. 
Consequently, even if the incidence plane of an optical receptor, such as 
photodiode 31, becomes very small, optical signals inside the 
cable-adapter 2 become intensified as they approach the small diameter 
face 2b. 
Reduced (decreased) optical losses enable an efficient light connection. To 
efficiently bind a photodiode 31, designed for receiving a high speed 
light signal (e.g., over the order of several hundred Mbps in particular), 
a step-index type fiber optic cable 1, developed for high-speed light 
communications, may be used as the transmission medium. By adding a 
frusto-conical cable-adapter 2 to its edge, the two parts are efficiently 
bound and the subsequent optical loss is minimized. 
Further, when adhering (affixing) the cable-adapter 2 to the light-exit 
end-face 1a of the fiber optic cable 1, the soft silicone elastomer 24 can 
be used for binding them. Handling thus becomes easy. 
FIGS. 8 and 9 illustrate the optical transmission device according to a 
second embodiment. Elements in the second embodiment that are 
correspondently functional to elements in the first embodiment are given 
the same reference numbers. 
In the first embodiment of the present invention, cable-adapter 2 is 
affixed to the light-exit end-face 1a of a known graded-index type optical 
fiber (fiber optic) cable 1 via the soft silicone elastomer 24. In the 
device according to the second embodiment, one end of a known plastic 
optical fiber is drawn directly from the plastic optical fiber cable 1 and 
the cable-adapter 2. This type of device may be manufactured by drawing 
the cable by virtue of a weight, as shown in FIG. 5. 
In the second embodiment, the optical fiber cable 1 and the cable-adapter 2 
comprise a core 11 and 21, respectively, and a clad portion 12, 22. The 
external cylindrical surface of the clad 12 of the cable 1 is covered with 
a coating 13. Likewise, the external cylindrical surface of the clad 
portion 22 of the cable-adapter 2 is painted with a low refractive index 
film 23. This configuration is similar to the first embodiment. However, 
in the second embodiment, the cable-adapter 2 is formed by drawing an end 
of the plastic optical fiber. The soft silicone elastomer 24 is therefore 
not used in this embodiment. The other constructions are the same as those 
of the first embodiment. 
Table 2 illustrates the light-reception sensitivity of varying diameters of 
the cable-adapter 2, obtained with the device according to the second 
embodiment. The experimental conditions are the same as in the first 
embodiment represented in Table 1. 
TABLE 2 
______________________________________ 
Small face of the cable-adapter 
(external diameter) Light reception sensitivity 
______________________________________ 
250 .mu.m -15.305 dBm 
300 .mu.m -14.217 dBm 
350 .mu.m -14.399 dBm 
400 .mu.m -13.298 dBm 
450 .mu.m -13.157 dBm 
500 .mu.m -12.450 dBm 
550 .mu.m -12.288 dBm 
600 .mu.m -12.284 dBm 
650 .mu.m -11.783 dBm 
700 .mu.m -11.170 dBm 
750 .mu.m -11.116 dBm 
(without tapered surface) 
______________________________________ 
As mentioned in Table 2, when the light leaving the small diameter face 2b 
of the cable-adapter 2 is received by a photodiode having, for instance, a 
diameter of 250 .mu.m, the light reception sensitivity measures -15.305 
dBm. Compared with the device without the taper of the second embodiment, 
there is a reduction (decrease) of only approximately 4.189 dBm. The loss 
is greatly reduced with respect to the value of 9.54 dBm obtained for the 
prior art example. Accordingly, as in the first embodiment, the 
light-reception sensitivity is improved vis-.alpha.-vis the results in the 
prior art, when using a photodiode 31 with a small surface chip for high 
speed transmission. 
In the first embodiment, the cable-adapter 2 is bound to the light-exit end 
face 1a of a known optical-fibercable 1 through the soft silicone 
elastomer 24. According to the second embodiment, the optical-fiber cable 
1 and the cable-adapter 2 are integrally formed by a drawing operation. 
In a third embodiment of the present invention, the same cable-adapter 
(cable-adapting device) 2 as in the first embodiment is placed at the 
light-exit end-face 1a of a known step-index type plastic optical fiber 1. 
The cable-adapters are housed into the ferrule 3 and properly positioned. 
However, the soft silicone elastomer 24 is not used in the third 
embodiment. Except for that point, all the other structural features are 
the same as those of the first embodiment. 
The light reception sensitivities, measured for the above device is 
represented in Table 3. The experimental conditions were the same as for 
the first embodiment. 
TABLE 3 
______________________________________ 
Small face of the cable-adapter 
(external diameter) Light reception sensitivity 
______________________________________ 
250 .mu.m -16.003 dBm 
300 .mu.m -15.571 dBm 
350 .mu.m -15.379 dBm 
400 .mu.m -15.460 dBm 
450 .mu.m -15.100 dBm 
500 .mu.m -14.219 dBm 
550 .mu.m -14.004 dBm 
600 .mu.m -13.213 dBm 
650 .mu.m -13.112 dBm 
700 .mu.m -12.522 dBm 
750 .mu.m -11.116 dBm 
(without tapered surface) 
______________________________________ 
As shown in Table 3, when the light leaving the small diameter face 2b of 
the cable-adapter 2 is captured by a photodiode having, for instance, a 
250 .mu.m diameter, the light-reception sensitivity amounts to -16.003 
dBm. This means that the sensitivity is reduced (lowered) by only 4.887 
dBm compared to that obtained when using a device without the taper. 
Compared to the 9.54 dBm reduction (decrease) in the prior art, the loss 
is greatly reduced. Compared to the first embodiment, the optical loss due 
to bonding is increased by a degree corresponding to the absence of the 
soft silicone elastomer 24. However, when a photodiode 31 is used with a 
small surface chip for high speed transmission, this sensitivity is 
greatly improved, as compared to the prior art. 
In embodiments 1 to 3, the fiber optic cable 1 belongs to a step-index type 
cable. Therefore, the cable-adapter 2 comprises a core 21 having a uniform 
refractive index. Alternatively, in the device according to a fourth 
embodiment, a graded-index type (distributed refractive index type) fiber 
optical is used, in which the refractive index varies inside the core of 
plastic optical-fiber cable 1. 
FIG. 10 illustrates an optical transmission device according to the fourth 
embodiment. The same reference numbers are used for the same functional 
elements described in the previous embodiments. In the present embodiment, 
a graded-index type plastic optical-fiber cable is used as the fiber optic 
cable 1 and the cable-adapter 2. The cable-adapter 2 is prepared in the 
same manner discussed with respect to the second embodiment, shown in FIG. 
5, by directly drawing the end portion of a known graded-index type 
optical-fiber. Therefore, the fiber optic cable 1 and the cable-adapter 2 
are integrally formed. Further, as in the second embodiment, a low 
refractive index film 23 is formed (painted) on the clad 22 of the 
cable-adapter 2. The graded-index type plastic optical fiber has a core, a 
diameter of which accounts for about 70% of the diameter of the optical 
fiber. The remaining 30% of the diameter of the optical fiber comprises a 
surrounding clad. 
The same ratios (discussed above with respect to the first embodiment) of 
the core and clad 11 and 21, respectively, of the optical fiber cable 1, 
to the core and clad 12 and 22, respectively, of the cable-adapter 2 are 
maintained in the fourth embodiment. 
In the graded-index type optical fiber cable 1, a diameter of the clad 12, 
a difference in specific refractive index, and a predetermined 
distribution coefficient of the refractive index are designated as 2a, 
.DELTA., and .alpha. (.congruent.2), respectively. A refractive index 
n(r), at a point located at a distance r from a central axis, can be 
calculated by the following formula: 
##EQU2## 
According to the above equations, both in the graded-index type fiber optic 
cable 1 and the cable-adapter 2, the refractive index continuously 
decreases as the cores 11 and 21 approach the respective clads 12 and 22. 
According to the refractive index distribution shown in FIG. 10, only an 
axial beam, located in the center of the cores 11 and 21, advances 
linearly. The beams outside this central axis continuously change their 
direction and return to the central axis. 
Such a locus of the beams is observed not only in the fiber optic cable 1, 
but also in the cable-adapter 2. In the cable-adapter 2, the amplitude of 
the light wave becomes smaller, in proportion to a decrease in the 
diameter size. Light beams inside the fiber optic cable 1 converge as they 
approach the small diameter face 2b of the cable-adapter 2. Accordingly, 
as shown in FIG. 2, the optical signal loss is greatly reduced when the 
beams are targeted from the small face 2b of the cable-adapting means 2 to 
a photodiode 31 having a small surface chip. 
Light reception sensitivity measured for the device of the fourth 
embodiment is represented in Table 4. The experimental conditions are the 
same as described for the second embodiment, except for the use of a 
graded-index type fiber optic cable 1 and cable-adapter 2. 
TABLE 4 
______________________________________ 
Small face of the cable-adapter 
(external diameter) Light reception sensitivity 
______________________________________ 
150 .mu.m -16.283 dBm 
200 .mu.m -13.964 dBm 
250 .mu.m ****** 
300 .mu.m -13.442 dBm 
350 .mu.m -13.277 dBm 
400 .mu.m -12.596 dBm 
450 .mu.m -12.723 dBm 
500 .mu.m -12.373 dBm 
550 .mu.m -12.299 dBm 
600 .mu.m -12.431 dBm 
650 .mu.m -12.173 dBm 
700 .mu.m -12.071 dBm 
750 .mu.m -12.086 dBm 
(without tapered surface) 
______________________________________ 
As shown in Table 4, an optical signal that uniformly leaves a fiber optic 
cable having a 500 .mu.m diameter core may be received by a photodiode 31 
having a 200 .mu.m diameter chip. Calculated with respect to the surface 
ratio, only 16% of the original signals can be received. When converted 
into dBm, the signal level reduction (decrease) is: 
EQU -10.times.Log(0.16)=7.96dBm. 
In the graded-index type plastic optical fiber, the core accounts for about 
70% of the fiber diameter. Accordingly, to obtain a core diameter of 200 
.mu.m in the small-diameter face 2b of the cable-adapting means 2, the 
external diameter of the face 2b must measure approximately 300 .mu.m. In 
Table 4, when the external diameter of the small face 2b, 750 .mu.m, is 
compared with that of 300 .mu.m, the sensitivity in the latter is lower 
than in the former by 1.4 dBm. If the surface ratio is calculated, the 
light intensity must be lower than the original intensity by 7.96 dBm. As 
the decrease is confined to a mere 1.4 dBm, a gain of 6.56 dBm (7.96-1.4) 
is attained. 
FIG. 11 illustrates a device according to a fifth embodiment of the present 
invention. The reference numbers are the same as previously used. 
According to this embodiment, the graded-index type core is used for the 
plastic optical fiber cable 1 and the cable-adapter 2, as in the fourth 
embodiment. However, the fiber optic cable 1 and the cable-adapter 2 are 
prepared as separate pieces, and arranged in a proper position inside the 
ferrule 3. In addition, the cable-adapter 2 is prepared in a manner 
similar to the fourth embodiment, and cut out in a predetermined 
dimension. The cable-adapter 2 is made of the same material as that of the 
optical fiber cable 1, or of a similar type material having the same level 
of numerical apertures as that of the optical fiber cable 1. 
The light-reception sensitivities measured with the device of the fifth 
embodiment, are represented in Table 5. The experimental conditions are as 
previously described. 
TABLE 5 
______________________________________ 
Small face of the cable-adapter 
(external diameter) Light reception sensitivity 
______________________________________ 
150 .mu.m -18.211 dBm 
200 .mu.m -15.239 .mu.m 
250 .mu.m ****** 
300 .mu.m -14.387 .mu.m 
350 .mu.m -14.502 .mu.m 
400 .mu.m -14.669 .mu.m 
450 .mu.m -13.510 .mu.m 
500 .mu.m -13.732 .mu.m 
550 .mu.m -13.526 .mu.m 
600 .mu.m -13.637 .mu.m 
650 .mu.m -12.991 .mu.m 
700 .mu.m -13.007 .mu.m 
750 .mu.m -12.086 .mu.m 
(without tapered surface) 
______________________________________ 
As described in the fourth embodiment, the optical loss exhibited in the 
prior art can be theoretically estimated to be approximately 7.96 dBm. 
However, when the small face 2b, having an external diameter of 300 .mu.m 
(-14.387 dBm), is compared with the diameter of 750 .mu.m (-12.086 dBm), 
the former has an intensity lower than the latter by 2.301 dBm 
(14.387-12.086). When calculated by the surface ratio, the decrease must 
be 7.96 dBm with regards to the original intensity. Compared to that, the 
actual decrease is confined to 2.30 dBm. This embodiment incurs an optical 
loss due to the bonding during the course of the transmission, which is 
different from the fourth embodiment. However, it attains a gain of 5.66 
dBm (7.96-2.30), as compared to the prior art. 
Each of the disclosed embodiments employs a plastic fiber optical cable 
having a relatively large core diameter. As such, an object of these 
embodiments is to converge the beams leaving the plastic optical fiber. 
This method can also be applied in the same manner to a glass optical 
fiber having a relatively large core. 
Although the present invention has been described with reference to 
particular means, materials and embodiments, it is to be understood that 
the invention is not limited to the particulars disclosed and extends to 
all equivalents within the scope of the claims.