Apparatus for Aligning an Optical Fiber in an LED Package

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.

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.

This invention will be more fully understood from the following detailed descriptions of preferred embodiments thereof, together with the drawing in which the figures are not drawn to scale and in which:

FIG. 1 is a greatly enlarged section view of an LED package 30 with an optical fiber 61 in the chimney 55 of the support member 44 thereof;

FIG. 2 is a greatly enlarged section view of the LED Chip 35 in FIG. 1 with the end 61A of the fiber in the emitting well 37 thereof;

FIG. 3 is a front elevation view of apparatus utilized in faricating pigtailed LED assemblies;

FIG. 4 is a greatly enlarged front elevation view of the ferrule 80 in FIG. 3;

FIG. 5 is a top view of the ferrule taken along line 5--5 in FIG. 3; and

FIG. 6 is a bottom view of the ferrule taken along line 6--6 in FIG. 3.

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.