CA1143822A

CA1143822A - Mounting structure for light emitting diode package for optical fiber communications

Info

Publication number: CA1143822A
Application number: CA000329137A
Authority: CA
Inventors: Kouichi Minemura; Kohroh Kobayashi; Yoshinobu Hayakawa
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1978-06-06
Filing date: 1979-06-05
Publication date: 1983-03-29
Also published as: DE2922949C2; FR2428328B1; GB2026235B; DE2922949A1; GB2026235A; FR2428328A1

Abstract

ABSTRACT
A structure for mounting an LED detachably adjacent an end of an optical fibre is described. The LED is incorporated in a package comprising a base having a recess at one end and a flange at the other.
The LED is housed in the recess and no part of the LED or associated leads protrudes above the top surface of the base. A receptacle has a through passage formed with a wider portion at the bottom end thereof.
The LED package is inserted in the wide portion with the flange abutting the lower surface of the receptacle. An optical fibre is inserted into the narrow portion of the passage from the top until the ground end of the fibre abuts the upper surface of the LED package, the fibre then being secured in place by an encircling nut which threadably engages the receptacle. Because the fibre has a greater diameter than the recess in the LED package, the LED and other components in the process cannot be damaged by engagement with the fibre end.

Description

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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 -- , ., . '' , ~'' ' ' ' ' ~

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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.

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~ 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 ~ ,

3~32Z

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 ~

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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 -.

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' , 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 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.

'' ' ;' '' ' - ' 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 ,`

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 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.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A structure for mounting a light emitting diode 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 substantially 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 cylindrical bore portion, and wherein said cylindrical sleeve means received in said second cylindrical 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 mount but not the pellet and lead wire.

2. A mounting structure for a light emitting diode package according to claim 1 wherein said recess is U-shaped extending from the center of said base mount in a diametric direction.

3. A mounting structure for a light emitting diode package according to claim 1 wherein said recess is circular and concentric with said base mount.

4. A mounting structure for a light emitting diode package according to claim 1 wherein said recess is rectangular and disposed in the center of said base mount.

5. A mounting structure for a light emitting diode package according to claim 4 wherein said pellet has a plurality of light emitting surfaces.

6. A mounting structure for a light emitting diode package according to claim 1 wherein said recess is formed by disposing a spacer affording a predetermined opening on a cylindrical member having an evenly flat surface.

7. A mounting structure for a light emitting diode package according to any of claims 1 to 3 wherein a spherical lens is disposed on said light emitting surface as one of said elements to be fabricated in the recess.

8. A mounting structure for a light emitting diode package according to any of claims 1 to 3 wherein said elements as fabricated in the recess are molded with a transparent resin.

9. A mounting structure for a light emitting diode package according to any of claims 1 to 3 including a cap nut threadably engaging the external threaded portion of the hollow cylindrical member and a resilient gasket dis-posed between a fire end of the hollow cylindrical member and the cap nut, the cap nut and the gasket arranged to receive therethrough the cylindrical sleeve means, whereby tightening of the cap nut causes the gasket to deform resiliently and grip the cylindrical sleeve means.