Pigtailing optical fiber

A method of aligning the optical axes of an optical fiber and a waveguide imbedded in an integrated optical chip and of attaching the fiber to the chip includes the step of coating the outer surface of the fiber with solder in the vicinity of an end of the fiber. A portion of the fiber at the coated end is cut away such that a flat surface at a tangential or near tangential relation to the fiber core results. A surface of the chip has solder pads disposed thereon such that the outer edges of the cut portion of the fiber contacts the solder pads when the flat surface of the fiber is placed in contact with the surface of the chip, such placement of the fiber onto the chip providing for vertical alignment of the optical axes of the fiber core and the waveguide in the chip. Finally, the solder on the outer fiber surface and on the solder pads is heated at the junction therebetween, the resulting surface tension forces bringing the optical axis of the fiber core into precise optical alignment with the optical axis of the waveguide in the chip.

TECHNICAL FIELD 
This invention relates to integrated optics, and more particularly to the 
alignment and connection of an optical fiber to a waveguide in an 
integrated optic chip. 
BACKGROUND ART 
In the art of mounting of optical fibers to integrated optic (IO) chips, it 
is critical that the waveguide formed or imbedded in the IO chip be in 
precise alignment with the fiber before affixing the fiber to the chip. 
Due to the typically small diameters (e.g., 0.005 to 0.01 mm) of both the 
fiber and waveguide, a small amount of misalignment therebetween may 
result in significant coupling loss of optical energy. 
It is known in the prior art to use support blocks with V-grooves formed 
therein to support the fiber and provide a coarse lateral and angular 
alignment of the fiber and waveguide. However, this approach does not 
provide the desired precise alignment of the fiber and waveguide before 
the two are affixed together. 
It is also known in the prior art to affix a support block to the IO chip 
and attach the fiber with adhesive to both the chip and block. An 
illustration of this is in U.S. Pat. No. 4,744,619 to Cameron. However, 
this method is inefficient in achieving the required precise optical 
alignment between the fiber and IO chip before affixation of the fiber to 
the chip. This is because Cameron teaches the use of elaborate 
micropositioner and imaging equipment apparatus for achieving alignment. 
The use of external positioners has a further drawback in that the 
positioner does not allow simultaneous positioning of closely spaced 
optical fibers at the output of the IO chip. Additionally, the use of 
external positioners is labor and capital intensive. 
DISCLOSURE OF INVENTION 
Objects of the present invention include the provision of an inexpensive 
and time saving method for interfacing an optical fiber to an IO chip such 
that the optical axes of the fiber and a waveguide on the IO chip are in 
precise alignment. 
According to the present invention, a method of aligning the optical axis 
in the core of an optical fiber with the optical axis in a waveguide 
imbedded in an integrated optical chip and of attaching the fiber to the 
chip includes the step of coating the outer surface of the fiber with 
solder in the vicinity of an end of the fiber; a portion of the fiber at 
the coated end is cut away such that a flat surface at a tangential or 
near tangential relation to the fiber core results, the relation of the 
flat surface to the fiber core being determined by optimal coupling of 
light energy between the fiber core and the waveguide; a surface of the 
chip has solder pads disposed thereon such that the outer edges of the cut 
portion of the fiber contact the solder pads when the flat surface of the 
fiber is placed in contact with the surface of the chip, such placement of 
the fiber onto the chip providing for vertical alignment of the optical 
axes of the fiber core and the waveguide in the chip; the solder on the 
outer fiber surface and on the solder pads is heated at the junction 
therebetween, the resulting surface tension forces bringing the optical 
axis of the fiber core into precise optical alignment with the optical 
axis of the waveguide in the chip. 
The method of the present invention has utility in achieving alignment of 
an optical fiber to a waveguide in an IO chip without the use of prior art 
external positioning equipment and apparatus to measure the optical 
transmission through the fiber/IO chip connection. The present invention 
is also inexpensive and time-saving, key factors in being able to produce 
IO devices such as fiber optic gyros in large quantities on a commercial 
basis.

BEST MODE FOR CARRYING OUT THE INVENTION 
In FIG. 1 is illustrated a perspective view of an optical fiber 10. The 
fiber 10 typically comprises a commercially available, single mode fiber, 
having a diameter in the range of, e.g., 0.005 to 0.01 mm. The fiber has 
an inner core portion 12, through which light is constrained to travel by 
an outer cladding portion 14. 
Initially, a portion of the fiber at an end is prepared by treating the 
outer surface 16 of the fiber with a known plating or wetting agent, e.g., 
such as an evaporative metal applied by known electron beam techniques. 
Next, the outer surface 16 of the fiber is coated with solder using the 
same electron beam techniques. The fiber is then attached to a substrate 
(not shown), such as a silicon wafer, to protect the fiber from cracking 
or breaking during the subsequent cutting process described hereinafter. 
The substrate itself has a layer of solder applied thereto. The solder on 
both the substrate and the fiber is melted using, e.g., known microwave 
soldering apparatus. 
In FIG. 2 is illustrated a perspective view of the fiber 10 of FIG. 1 
prepared in accordance with the present invention. A portion of the fiber 
at the prepared end is cut away from the remainder of the fiber such that 
a flat surface 18 results. The depth of cut may be, e.g., to just below 
the fiber core such that the flat surface 18 is tangent to the core. The 
saw cut is accomplished by, e.g., a fine grit diamond abrasive dicing saw 
blade which produces a polished, optically smooth surface 18. The depth of 
cut is controlled by, e.g., injecting a light beam from one end of the 
fiber into the core from a light source (not shown). The light beam in the 
core is monitored during the aforementioned cutting process by a light 
sensing device (not shown), such as a photodiode, to determine the proper 
depth of cut. 
However, it is to be understood that the flat surface 18 does not have to 
be in a tangential relationship to the fiber core; instead, the flat 
surface may be in a near tangential relationship to the core. This would 
occur if the cut was made to a depth either somewhat beyond the core or 
not entirely through the core. In any event, the resulting relation 
between the flat surface and the core is determined by optimal coupling of 
optical energy between the core and the waveguide. This optimal coupling 
is determined experimentally prior to execution of the method of the 
present invention. As an example, an optical fiber of similar type and 
kind to that cut in accordance with the present invention may have its 
core cut in a similar manner using the aforementioned dicing saw blade. 
The core of this experimental fiber is cut, however, in increments; all 
the while with the light through the core being monitored by the 
aforementioned light sensing device. The incremental cutting process is 
stopped when the desired amount of light extinction through the core is 
observed. This translates into the optimal coupling point. The resulting 
depth of cut of the core of the experimental fiber is then used for the 
depth of cut of the fiber in accordance with the present invention. 
It is to be noted that, for use with polarization maintaining fibers, the 
method of the present invention requires that these types of fibers be 
rotated into a desired orientation prior to cutting of the fiber. This is 
to align the extra ordinary axis of the fiber with the resulting flat 
surface cut into the fiber. With modern single mode fibers this extra 
ordinary axis can be visually observed with the naked eye or a microscope. 
Referring to FIG. 3, a surface 22 of an IO chip 24 is prepared by locating 
a pair of solder pads 26,28 thereon. The chip 24 may comprise, e.g., 
lithium niobate or lithium tantalate. The areas of the chip where each 
solder pad 26,28 is to be located are defined using standard lithographic 
techniques. These areas are plated with evaporative metal using, e.g., the 
aforementioned electron beam deposition technique. A metered amount of 
solder is then applied to the plated areas using the electron beam 
deposition technique. The plating confines the extent to which the solder 
is placed on the chip. 
The standard lithographic techniques used to locate the solder pads make 
possible the accurate alignment of the pads with respect to a waveguide 30 
on the chip. The pads are located on the chip such that the outer edges of 
the flat surface 18 of the fiber 10 are in contact with the pads 26,28 
when the fiber is ultimately brought in contact with the IO chip, as 
described hereinafter with respect to FIG. 4. 
Referring to FIG. 4, the fiber is assembled onto the IO chip such that the 
flat surface 18 of the fiber is in contact with the prepared surface 22 of 
the chip. More particularly, the outer edges of the flat surface of the 
fiber are in contact with the solder pads 26,28 on the surface of the 
chip. This provides the vertical alignment of the optical axis of the 
fiber core 12 with the optical axis of the waveguide 30 on the IO chip. 
The solder on the outer surface of the fiber at the junction of the fiber 
and the solder pads together with the solder on the solder pads is then 
melted using, e.g., microwave soldering apparatus. Since the flat surface 
of the fiber was not pretreated with solder, the molten solder will not 
wet this surface and come between it and the surface of the IO chip. As a 
result, precise horizontal alignment of the optical axes of the fiber core 
and IO chip waveguide is accomplished due to surface tension forces at the 
time of microwave soldering. 
The method of the present invention has utility in achieving alignment of 
the optical axes of an optical fiber and a waveguide in an IO chip without 
the use of prior art external positioning equipment and apparatus to 
measure the optical transmission through the fiber/IO chip connection. The 
present invention is also inexpensive and time-saving, key factors in 
being able to produce IO devices such as fiber optic gyros in large 
quantities on a commercial basis. 
As described hereinbefore, the optical fiber 10 is attached to a substrate 
to prevent the fiber from cracking or breaking during this cutting 
process. However, it is to be understood that this step of attaching is 
not required in the broadest scope of the present invention. Any method 
may be used, if desired, for preventing the fiber from breaking or 
cracking, in light of the teachings herein. 
Further, the plating of the fiber and chip has been described as being 
accomplished using evaporative metal applied with known electron beam 
techniques. However, it is to be understood that, for the broadest scope 
of the present invention, any appropriate type of a plating or wetting 
agent may be applied using any appropriate known method. 
All of the foregoing changes are irrelevant; it suffices for the method of 
the present invention that an outer surface of an optical fiber be coated 
with solder in the vicinity of an end thereof; a portion of the fiber at 
the coated end is cut away such that a flat surface at a tangential or 
near tangential relationship to the fiber core results; a surface of the 
chip has solder pads disposed thereon such that the outer edges of the cut 
portion of the fiber contacts the solder pads when the flat surface of the 
fiber is placed in contact with the surface of the chip, such placement of 
the fiber onto the chip providing for vertical alignment of the optical 
axes of the fiber core and the waveguide in the chip; the solder on the 
outer fiber surface and on the solder pads is heated at the junction 
therebetween, the resulting surface tension forces bringing the optical 
axis of the fiber core into precise optical alignment with the optical 
axis of the waveguide in the chip. 
Although the invention has been illustrated and described with respect to a 
best mode embodiment thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made without departing 
from the spirit and scope of the invention.