Apparatus for aligning an optical fiber in an LED package

Method and apparatus for aligning one end of an optical fiber in the emitting well of an LED employs an elongated rod having a flat milled halfway through the mid-section thereof for forming a recess and first and second channels extending between the recess and an associated end of the rod to the depth of the recess. The two channels are located in front of the bottom of the recess and are oriented at 90.degree. with respect to each other in an end view. The channels also overlap along the center line of the rod and are dimensioned for loosely-releasably receiving and supporting a length of fiber in a straight line while the one end thereof is aligned with the emitting surface of the LED.

BACKGROUND OF THE INVENTION 
This invention relates to fabricating pigtailed LED assemblies and more 
particularly to method and apparatus for aligning the end of an optical 
fiber in the concave emitting well in an LED. 
A light emitting diode (LED) is commonly employed as the source of light in 
the transmitter of a fiber optic communication system. In order to 
maximize the amount of light available for transmission in the fiber 
transmission lines of the system, an assembly is fabricated with a fiber 
pigtail precisely aligned in the emitting well of the LED. The practice is 
to secure the fiber to the side of a micro-manipulator with two tweezers 
that are vertically spaced approximately three inches apart. Each of the 
tweezers lightly clamps the fiber at what approximates a point contact, 
with the lower-one end of the fiber being adjacent the emitter surface of 
the LED. After the micro-manipulator is operated to obtain the maximum 
transmission of light in the fiber, an epoxy resin is poured into the 
chimney of a support member associated with the LED for fixing the 
orientation of it and the fiber. It has been found that there is normally 
a bow in the unsupported length of fiber between and below the two 
tweezers which makes it difficult to precisely align the one end of the 
fiber in the emitting well of the LED so as to obtain maximum transmission 
of light in the fiber. An object of this invention is the provision of 
improved method and apparatus for fabricating pigtailed LED assemblies. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a ferrule for loosely-releaseably 
supporting a filament over at least the major portion of a limited length 
thereof and holding the limited length of filament in a straight line so 
as to reduce bowing of the filament adjacent one end of the ferrule that 
is proximate the emitting surface of a light emitting element comprises: 
an elongated member having a length between opposite ends thereof that 
extends in a prescribed direction along a given straight line and which 
corresponds to the limited length; the member having a recess cut into the 
exterior surface thereof over its breadth for defining first and second 
sections on opposite sides of the recess and at opposite ends of the 
member, the recess extending over a prescribed length which is much less 
than the limited length and having a depth that is at least equal to the 
distance between the given line and the circumference of the member; the 
first and second sections having associated first and second channels 
therein extending over the lengths thereof along the given line and being 
in front of the bottom of the recess so as to open into the recess, the 
channels being dimensioned for receiving the largest filament that is to 
be supported and extending into the member to the given line; the two 
channels overlapping along the given line sufficiently for receiving a 
filament therein and being oriented in an end view of the member looking 
along the given line for forming an acute angle therebetween in the area 
of the recess that is sufficient for receiving and holding a filament, 
laid in the channels and extending across the recess, in a straight line 
over the length of the member. In a preferred embodiment, the recess is 
formed by a flat in a plane that includes the given line for causing the 
filament to contact and be supported by surfaces of the channels and the 
flat over the length of the member.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2, the LED package 30 comprises an 
electrically conductive header 34 having a pair of electrodes 38 and 40 
therein and a pedestal 42 thereon supporting a light emitting 
semiconductor chip 35, and a support member 44 that is adapted for holding 
an optical fiber 61 therein. The electrode 38 is electrically connected to 
the header 34, which may be made of copper. The electrode 40 extends 
through the header, however, and is dielectrically supported in it by a 
non-conductive epoxy cement 48 such as EPO-TEK H72 manufactured by 
EPO-TEK, Inc. A gold wire 50 is bonded to the LED chip 35 and to the end 
of electrode 40 for completing an electrical circuit therebetween. This 
makes it possible for the LED to pass electrical current and emit light 
from the well 37 therein (see FIG. 2) when the electrodes 38 and 40 are 
connected to a power supply. The support member 44 comprises a chimney 55 
that is coaxial with a much larger cylindrical sleeve section 56 thereof. 
The ID of the open end of the sleeve 56 is sized to slide smoothly over a 
circular projection 45 on the header until it rests on the shoulder 46. 
This centers the opening in the chimney over the emitting well in the LED. 
The sleeve 56 is bonded to the header with epoxy cement 57 for keeping the 
chimney centered over the LED. The sleeve 56 has openings 58 therein which 
let an operator see how close the end of fiber 61 is to the LED's emitting 
surface. 
A pigtailed LED assembly is fabricated by energizing the LED for causing it 
to emit light; passing the end 61A of an optical fiber 61 through the 
central opening through the chimney 55; precisely locating the end 61A of 
the fiber in the emitting well 37 and positioning it approximately 0.001 
inch from the emitting surface of the LED chip 35; locating epoxy cement 
in the opening in the chimney 55; and curing the epoxy cement while 
maintaining the fiber end 61A positioned in the emitting well for 
providing maximum transmission of light in the fiber. A shrink-fit tubing 
(not shown) is then placed over the fiber and chimney 55 for protecting 
the fiber. A cylindrical heat sink 62 is then placed over the circular 
flange 64 on the header and bonded to the latter, e.g. with an epoxy 
cement, prior to filling the cavity between the heat sink 62 and support 
member 44 with a thermally conductive epoxy such as EPO-TEK 930. 
Apparatus for fabricating a pigtailed LED assembly is illustrated in FIG. 3 
and comprises a table 70 supporting a micro-manipulator 72 and post 74 
adjacent an opening 76 in the table which receives an LED package 30; a 
vertically spaced apart ferrule 80 and tweezers 82 that are attached to a 
post 84 in the side of the micro-manipulator for supporting the fiber; and 
a tweezers 86 that is attached to the post 74 for supporting a loop 63 of 
excess fiber above the LED package 30. In accordance with this invention, 
the tweezers 82 fix the relative position of the fiber end 61A with 
respect to the top of the micro-manipulator and the ferrule 80 holds the 
length of fiber adjacent the end 61A in a straight line. The electrodes 38 
and 40 of an LED package 30 in the table opening 76 are connected to the 
power supply 86 for energizing the LED and causing it to emit light into 
the one end 61A of the fiber. A photodetector 88 and power meter 89 are 
connected to the other end 61B of the fiber for indicating the intensity 
of light transmitted through the fiber. 
Referring now to FIGS. 4-6 the ferrule 80 is essentially a brass rod 91 
having a flat 93 milled to a prescribed depth d in the central section 
thereof and a pair of longitudinal channels 96 and 97 milled to the same 
depth d in the upper and lower sections 94 and 95 thereof. The channels 
are located in front of the plane including the flat bottom 93 of the 
recess and have a width that is much greater than the diameter of an 
optical fiber 61. The channels are also orthogonal to each other in an end 
view of the ferrule (see FIGS. 5 and 6) and are oriented so that they 
overlap in front of the flat 93 by an amount that is much larger than the 
cross-sectional area of the fiber 61. Thus, there is a straight line such 
as the center line of the rod 91 that extends in front of the flat 93 and 
through the overlapping portions of the channels 96 and 97. The length of 
the flat 93 is much greater than the bending radius of an optical fiber 
61. The ferrule 80 is vertically mounted in the apparatus of FIG. 3 with a 
clamp 98 on the post 84 that firmly grips only the sides of the ferrule. 
This leaves the channel 96 exposed over its length (along the left side of 
the upper section 94 in FIGS. 3 and 4). 
In accordance with this invention, the fiber 61 in FIG. 3 is mounted in an 
LED package 30 in the table opening 76 (see FIG. 3) by pressing a length 
of the fiber 61 into the upper channel 96 of the ferrule 80, bending the 
fiber across the flat 93, and inserting the adjacent length of the fiber 
61 into the lower channel 97. Since the channels intersect in a right 
angle and the excess fiber at 63 is directly above the center line of the 
ferrule, the channels loosely-releaseably support the length of fiber 61 
that is in the ferrule in a straight line. After the end 61A of the fiber 
is inserted into the chimney 55 of the LED package, the tweezers 82 are 
made to lightly clamp the fiber and fix the relative position of the fiber 
end 61A in the micro-manipulator. The micro-manipulator is then operated 
to center the fiber end 61A over, and locate it approximate 0.001 inch 
above, the LED's emitting surface 37, i.e., to position the fiber end 61A 
for obtaining maximum transmission of light on the fiber. When a reading 
on meter 89 indicates optimum positioning of the fiber end 61A, epoxy 
cement is injected into the top of the chimney 55 until it forms a small 
meniscus at the bottom of the chimney that is inside the sleeve 56. If the 
intensity of the transmitted light decreases during curing of the epoxy 
cement, the position of the fiber end 61A may be moved to compensate for 
it. The LED package and connected fiber 61 are removed from the apparatus 
in FIG. 3 by releasing the clamps 82 and 86 and removing the fiber from 
the channels in the ferrule prior to threading a heat shrinkable 
protective tubing (not shown) over the fiber and chimney 55 of the support 
member 44, attaching the heat sink 62 to the pigtailed package 30, and 
filling the cavity formed between the heat sink and support member with a 
thermally conductive epoxy. 
In apparatus for practicing this fabrication method that was built and 
satisfactorily operated for manufacturing pigtailed LED assemblies with a 
1 meter length of 0.002 inch diameter optical fiber, the ferrule was 
fabricated from a 2.75 inch length of a 0.250 inch diameter half-hard 
brass rod. The flat 93 was 0.75 inch long, d=0.130 inch, and the channels 
were 0.014 inch wide and one inch long. The tweezers 82 was located 1.5 
inches above the end 101 of the ferrule and was caused to clamp the fiber 
when it was located in the channels and the chimney 55 and approximately 
0.125 inch above the LED. The chimney had a 0.028 inch ID, a length of 
0.120 inch, and was spaced 0.60 inch above the semi-conductor chip 35. The 
intensity of light transmitted through the fiber in the apparatus in FIG. 
3 was found to change very little during curing of epoxy filler that was 
placed in the fiber support chimney. 
Although this invention is described in relation to preferred embodiments 
thereof, variations and modifications will occur to those skilled in the 
art. By way of example, the ferrule 80 may have a circumference that is 
irregularly shaped or that is a regular shape other than a circle, e.g., 
it may be a rectangular bar or an elliptically shaped rod. Also, the 
recess may be formed by other than milling and the bottom thereof may be 
other than flat. Additionally, the depth of the recess may be greater than 
that of the channel 96. Further, the flat 93 and channels do not have to 
be centered in the rod 91. Also, the channels may intersect at angles of 
other than 90.degree., although the angle of intersection must be 
sufficient to hold the fiber 61 in a straight line in the ferrule and 
preferrably be less than 180.degree.. The scope of this invention is 
therefore defined by the appended claims, rather than the preceeding 
detailed descriptions.