Note: Descriptions are shown in the official language in which they were submitted.
2'~
This invention rel~tes to semiconductor light emitting diode pack-
ages used as light sources in oytical fiber communication systems and, more
particularly, to a mounting structure for such a light emitting diode package
that can be detachably coupled to an optical fiber with high coupling
efficiency and without damaging the light emitting diode pellet ~hereunder
referred to as an LED pellet) or a lead wire serving as an electrode.
Optical fiber communication systems which use an optical fiber
as a transmission path have several advantages: wide band width trans-
mission, high immunity to elertromagnetic induction, a longer repeater
spacing and easy installation of a transmission path because of the flex-
ible and light weight optical fiber. After many years of experimental work,
optiral fiber communication systems have developed to a stage where a
commercial system can be operated. Of several available optical fiber com-
munication systems, a simple, low-speed, short-distance transmission system
which operates at a bit rate of less than several tens of megabits per
second and at a repeater spacing of less than one kilometer is expected to
be in great demand as a system for data transmission in a building or between
computers.
A light emitting diode which is employed in such simple optical
data transmission is required not only to have appropriate optical and
electrical characteristics but also to incorporate an inexpensive structure
~hat permits direct coupling to an optical fiber. One example of such
structures that have been proposed to date is disclosed in a paper entitled
"High-Efficiency Long-Lived GaAs LED's for Fiber-Optical Communications",
by M. Abe et al, printed on pages 990 to 994, IEEE TRANSACTIONS ON ELECTRON
DEVICES, Vol. ED-24, No. 7, July 1977. This prior art reference shows a
structure wherein a cap with a spherical-ended fiber covered with a metal
sleeve is fixed through bonding to a stem by an epoxy resin so that the fiber
-- 1 -- , .,
. '' , ~'' ' ' ' ' ~
'
2~
rests right above all LED pellet mounted on the stem. However, the structure
involves a variation in the thickness oE the epoxy resin layer between the cap
and stem in a range of from 100 to 300 microns. This inevitably results in
unstable spacing between the fiber end and the upper surface of the LED pellet,
thus causing variations in tile coupling efficiency between the LED pellet and
the optical fiber. Furthermore, since the fiber end extends beyond the top
of the cap, the spherical lens formed at the tip of the fiber may contact and
damage the lead wire as the cap is adjusted for its position. As a further
disadvantage, the structure cannot be detached from the optical fiber. On
the other hand, a detachable structure is disclosed in a paper entitled
"High-Performance Coupler with Connector for Optical Communication" by
Takahashi et al., published on 4 - 38, 1978 National Convention Record of
The Institute of Electronics and Communication Engineers of Japan, March 5,
1978. The structure comprises a receptacle on which are mounted an LED
package and a light focusing transmission body having a distribution of
refractive index such that it decreases in proportion to the square of the
distance from the central axis. However, the light focusing transmission
body is costly and the mounting structure occupies a large space.
It is therefore an object of this invention to provide an inex-
pensive mounting structure for a light emitting diode which assures direct,correct and easy coupling of an LED pellet to an optical fiber without damaging
the pellet or cutting a lead wire serving as an electrode.
It is another object of this invention to provide a mounting
structure for a light emitting diode which can be detached from an optical
fiber and which can be connected to the fiber with improved coupling efficiency
in such a manner that the distance between the fiber and the light emitting
surface of the diode is minimized.
5~ A
- 2 -
~ ccording to the present invention, there is provided a structure
for mounting a light emitting cliode so as to couple said diode with an optical
fibre, the structure comprising a light emitting diode package including a
cylindrical base mount which has a recess affording a predetermined opening
and depth in the flat portion on one side and a flange on the other side, at
least one electrode terminal which is electrically isolated from said base
mount and extends beyond said base mount on the recessed side by a length
shorter than the depth of said recess, an electrode terminal extending downward
from said flange and electrically connected to said base mount, a pellet of
light emitting diode thinner than the depth of said recess resting in the
recess and having a light emitting surface, and a lead wire for electrically
connecting said pellet with said insulated electrode terminal~ said isolated
electrode terminal, pellet, lead wire and other elements to be fabricated being
so fabricated that they are confined in said recess without protruding from the
flat portion of said base mount; and a receptacle including a base plate and
a hollow cylindrical member connected thereto, said base plate having a first
cylindrical bore portion of an inner diameter slightly larger than the outer
diameter of said cylindrical base mount, said hollow cylindrical member having
an external threaded portion and a second cylindrical bore portion connected
to said first cylindrical bore portion, the second cylindrical bore portion
being slightly larger in diameter than a cylindrical sleeve means for sub-
stantially coaxially holding an end portion of the optical fibre, wherein said
package is fixed at least partially in said first cylindrical bore portion with
the light emitting surface of said pellet facing toward said second cylindri-
cal bore portion, and wherein said cylindrical sleeve means received in said
second cylindrîcal bore portion is larger in diameter than the opening of
said recess so as to prevent said cylindrical sleeve from entering said recess
and engages the flat portion of the base
~ ,
--3--
3~32Z
mount but not the pellet and lead wire.
Other purposes and advantages of this invention will be apparent
by reading the following description of the invention in conjunction with the
accompanying drawin~s in which:
Figure 1 is a cross-sectional view of one embodiment of this
invention;
Figures 2a and 2b are a plan v:iew and cross-sectional view, re-
spectively, of a light emitting diode package employed in the embodiment of
this invention;
Figures 3a and 3b are a plan view and cross-sectional view,
respectively, of another light emitting diode package employed ln the
embodiment of this invention;
Figures 4a and 4b are a plan view and cross-sectional view,
respectively, of still another light emitting diode package employed in the
embodiment of this invention;
Figure 5a is a simplified cross-sectional view of a mounting
structure of light emitting diode package according to another embodiment of
this invention; and
Figure 5b is a plan view of such package as seen from the plane
A-A'.
Referring to Figure 1, the embodiment shown therein has a receptacle
90 and a light emitting diode package 200. The receptacle 90 comprises a base
plate 93 which is an integral part of a vertically extending cylindrical
member 94 which is provided along its center with a first hollow or bore
portion 91 having an inner diameter of 5.0 mm and a second hollow or bore
portion 92 connected thereto and having an inner diameter of 2.01 mm. The
light emitting diode package 200 is inserted into the first hollow portion 91
~ 4 ~
.
with the light emitting surface up, and is fixed to the base plate 93 after
it has been so adjusted that the light emitting surface will rest in a sub-
stantially central location. As will be described hereunder in detail with
reference to Figures 2a and 2b, the light emitting diode package is so
fabricated that an LED pellet 1, a lead wire 5 and an electrode terminal 3
will be confined in a recess 2a without protruding from the upper end surface
18 of the package. The inner diameter of t:he second hollow portion 92 that is
connected to the first hollow portion of the receptacle is larger than the size
of the opening of the recess 2a (see Figure 2a~. The terminating portion 12
of an optical fiber 10 having a core diameter of 100 microns, and which is
reinforced with a metal sleeve 11 having an outer diameter of 2.0 mm and which
has its end surface ground optically is inserted into the second hollow portion
92,
Cylindrical member 94 is externally threaded and receives a cap
nut 13 in threaded engagement therewith. The cap nut 13 has a central aperture
13a through which sleeve 11 extends. An annular resilient gasket 13b is
received between the upper end of member 94 and cap nut 13 and on tightening
of nut 13, gasket 13b is deformed radially inwardly against sleeve 11 to hold
it securely in position in
- 4a -
.
, .
' ,
3~22
such a manner that the fiber is fixed to the upper end surface 18 of the
package 200 in contact relationship. According to the package mounting
structure 300 shown in Figure 1, the terminating portion 12 of the optical
fibcr 10 has an outer diameter larger than the size of the opening of the
recess 2a and therefore will not contact or damage the LED pellet and lead
wire disposed in the recess. In addition, the LED pellet can always be
spaced at a constant distance from the end surface o the optical fiber, thus
eliminating the chances of causing a variation in the coupling efficiency.
Furthermore, due to free detachability from the terminating portion of the
optical fiber, the light emitting diode package of this invention can easily
be installed on an optical fiber communication equipment.
The light emitting diode package used in this invention will be des-
cribed in more detail, referring to Figures 2a and 2b. ~ stem 100 is composed
of a base mount 2 comprising a gold-metalized steel cylinder 2d having a dia-
meter of 4.5 mm plus a flange 2b, electrode terminals 3 and 4 each having a
diameter of 0.4 mm, and an insulating member 6 having a diameter of 1 mm for
electrically isolating the electrode terminal 3 from the base mount. The metal
cylinder 2d has a U-shaped recess 2a which is 300 microns deep from the upper
end surface 18, one millimeter wide, and 2.5 mm long in a diametr~ic direction.
The recess may be said to be U-shaped, opening out to a side of the cylinder
2d and being curved at its inner surface. In the recess and in the center of
the metal cylinder is disposed the LED pellet 1 having a dimension of 300
microns in width, 700 microns in length and 150 microns in thickness with the
light emitting surface of a diameter of 80 microns facing up. The pellet is
fitted to the base mount by fusion of a gold-tin alloy to establish an electri-
cal connection with the electrode terminal 4. The layer of the alloy is only
about 10 microns thick and so it will not cause a variation in the distance
between the light emitting surface of LED and the upper end surface of the
-- 5 --
3~
base mount. The electrode terminal 3 which protrudes from the bottom of the
recess 2a is electrically connectcd to the pellet 1 ~y a lead wire having a
diameter of S0 microns and serving as an elcctrode. The electrode tcrminal
3 is so fabricated tha~ it will not extend beyond the upper end surface 18
of the base mount.
Referring again to Figure 1, the sleeve ll of the optical fiber
has an outer diam~ter of 2.0 mm whereas the recess 2a in the base mount is
one millimeter wide, and therefore, the terminating portion of the optical
fiber will not damage any of the elements fabricated in the recess. Experi-
mental data show that a light emitting surface separated from the terminal end
of an optical fiber by a distance of 300 microns results in a coupling loss
of 3 dB more than when the distance is zero, and that the loss increases ex-
ponentially with distance. However~ according to the light emitting diode
package of this invention, the distance can be reduced to about 50 microns,
giving an increase of only 1 to 2 dB in coupling loss upon coupling to the
optical fiber 10.
Referring now to Figures 3a and 3bJ the base mount 2 of the light
emitting diode package 200 comprises a cylinder 2d having an evenly ~lat
surface and a diameter of 4.5 mm, and a recess 2a is formed by spot welding
to the cylinder 2d a stainless steel C-shaped spacer 7 having an inner dia-
meter of 1.5 mm, an outer diameter of 4 mm and a thickness of 300 microns.
The spot welding is performed by a YAG laser. The LED pellet 1 is fitted to
the base mount by a fused layer of a gold-tin alloy in such a manner that the
light emitting surface of the pellet is in a substantially central portion of
the recess, and is electrically connected to an electrode terminal 4. The
upper surface of the pellet is connected to an electrode terminal 3 by a lead
wire 5. A description o~ the dimensions of these elements is omitted because
they are the same as those described in connection with Figures 2a and 2b.
- 6 -
'' ' ;' '' ' - '
Referring now to Figures 4a and 4b, the base mount 2 of the light
emitting diode package 2Q0 is composed of a cylinder 2d having an outer dia-
meter of 4 mm and a flange 2b. In the center of the cylinder 2d are disposed
a circular recess 2a one millimeter deep having an inner diameter of 1.5 mm
and a flat bottom as well as rectangular recesses 2c and 2c' connected to
the circular recess 2a. Electrode terminals 3 and 3' are electrically in-
sulated from the base mount by insulator members 6 and 6', and fixed in such
a way that they protrude from the bottom of the recesses 2c and 2c' by a
distance of 500 microns. The electrode terminals 3, 3' and 4 extend downward
from the base mount. An A1203- insulator plate 14 which is 800 ~icrons long,
500 microns wide and 500 microns thick and which has its upper surface
metalized with gold is fitted to the bottom of the recess 2a by a fused layer
of a gold-tin alloy. To the insulator plate is fitted by using a gold-tin
alloy an LED pellet measuring 300 microns in width, 500 microns in length
and 150 microns in thickness and having a light emitting area of 50 microns
across. A spherical lens lS having a diameter of about 200 microns which is
prepared by fusing a glass fiber is fixed by a transparent epoxy resin to
the upper surface of the light emitting area in contact relationship. The
upper end of an electrode terminal 3 is electrically connected to the LED
pellet 1 by a lead wire 5 having a diameter of 50 microns~ and the upper end
of an electrode terminal 3' is electrically connected to the LED pellet 1
by both the metalized gold layer on the insulator plate 14 and a lead wire
17 having a diameter of 5Q microns. Therefore, the upper end surface 18 of
the base mount has a clearance of about 150 microns from the spherical lens
15, so that none of the lens 15, LED pellet 1, electrode terminals 3, 3'
and the lead ~ires 11 and 17 ~ill protrude from the recess 2a, 2c or 2c'.
When the package illustrated in Figures 4a and 4b is installed in the recep-
tacle 90 of Figure 1 and coupled to the terminating portion of an optical
- 7 -
,`
3~;~2
fiber, the resulting increase o~ optical coupling loss i5 less than 0.5 dB
which is far smaller than when the optical fiber is disposed in contact with
the upper surface of the spherical lens 15. The improvement of coupling
efficiency due to the use o~ a spherical lens is conspicuous as the diameter
of the light emitting surface becomes even smaller than th~ core diameter of
the optical fiber; the embodiment illustrated in Figures 4a and 4b achieves
an improvement of 3 dB in coupling efficiency.
Referring to Figures 5a and 5b, the base mount 2 has in its center
a rectangular recess 2a, which is 2.5 mm wide, 3.5 mm long and one millimeter
deep. Within the recess is disposed a gold-metalized insulator plate 14,
one millimeter wide, 2.5 mm long and 0.5 mm thick, and an LED pellet, 0.8 mm
wide, 2.1 mm long and 0.15 mm thick is fitted to the insulator plate by
fusing a gold-tin alloy. In this pellet are defined six light emitting
areas at an interval of 300 microns that form a corresponding number of LEDs,
and spherical lenses 41 to 46 are mounted in such a manner that they contact
the upper surfaces of the respective light emitting areas. The upper elec-
trodes of the light emitting diodes are electrically insulated from each
other and are electrically connected to electrode terminals 51 to 56 by lead
wires 61 to 66, respectively. The LED pellet 1 has a lower electrode which
is common to the light emitting diodes and electrically connected to an
electrode terminal 3' by both the metalized gold electrode on the upper
surface of the insulator plat0 14 and a lead wire 17. These electrode ter-
minals are electrically isolated from the base mount 2 by an insulator
member 6. An electrode terminal 4 is directly connected to the base mount
2. The ends of optical fibers 71 to 76 are spaced ifrom each other at the
distance by which the light emitting surfaces of the light emitting diodes
are spaced from each other~ and are protected by a metal sleeve 11 ~3 mm x
4 mm). The surfaces of these ends are disposed above the recess 2a in the
-- 8 --
base mount 2 in close proximity with the upper end surface 18 of the base
mount. In Figures 5a and 5b, a receptacle and a plug are omitted for simp-
licity. In the illustrated embodiment, the central axes of the output beams
of the spherical lenses 41 to 46 are in substantial alignment with those
of the optical fibers 71 to 76, and so, the output beams of the six light
emitting diodes can be coupled effectively to the optical fibers 71 to 76,
respectively. In additionl the dimensions of the sleeve 11 and the recess
2a are so adjusted that the sleeve will not enter the recess, and as a
result, neither the sleeve nor the optical fibers 71 to 76 will possibly
contact the spherical lenses 41 to 46.
It is finally to be noted that for reinforcement of the fabri-
cated LED pellet, lead wires and spherical lenses and for hermetic sealing
of the light emitting diode package, the recess in the base mount is pre-
ferably filled with a transparent epoxy resin, acrylic resin or silicone
resin.
_ g _
' ' ` .
,
.
