Note: Descriptions are shown in the official language in which they were submitted.
1 335870
METHOD OF MAKING AN ECONOMICAL FIBER COUPLER
T~ ~ adiv~ion~ ofC~n~ n PatentA~p~ca~on Sen~ No.
572,919 ~edJuly25,1988.
Background of the Invention
Certain types of fiber optic systems require couplers
in which at least a portion of the light propagating in an
optical fiber is coupled to one or more output fibers. The
present invention relates to such fiber optic couplers and
15 more particularly to a cost effective and reproducible
method of making such fiber optic couplers.
It has been known that coupling occurs between two
closely spaced cores in a multiple core device. The
coupling efficiency increases with decreasing core
2 separation and, in the case of single-mode cores, with
decreasing core diameter. A number of couplers that are
based on these principles have been developed. Such
couplers are capable of low loss operation; they typically
exhibit an excess loss of about 1 dB or less.
Multimode and single-mode couplers have been formed by
positioning a plurality of fibers in a side-by-side
relationship along a suitable length thereof and fusing the
- - 2 - 1 3 3 5 8 7 0
claddings together to secure the fibers and reduce the
spacings between the cores. Coupllng can be enhanced by
stretching and by rotating the fibers along the fused
length thereof as taught in U.S. Patent Number ~,426,215 to
S Murphy; however, rotatlng the fibers is dlsadYantageeous
for certain purposes. Also, a portion of the cladding ~s
sometimes removed by etch~ng or grlnding to decrease the
intercore distance as taught in U.S. Patent ~umber
4,449,781 to Lightstone et al. Since the coupling region
is fragile and is exposed to the atmosphere, such couplers
st then be provided with a hermetic enclosure. These
processes are labor intensive and therefore expensive, they
may lac~ long term integrety, and do not always result in
couplers exhibiting predetermined desired coupling
characteristics. Such disadvantages are particularly
app~rent in the manufacture of certain single-mode couplers
w~,erein the couplinq core sections are to remain parallel
to each other to i-.s~re that the propagation constants are
matched and of certain single-mode couplers which must
possess optical characteristics such as polarization
~eten.ion.
Although mos~ couplers are made by applying heat
cirectly to the f_bers to be joined, U.S. Patent ~umber
3,579,316 to Dyott et al. teaches a method wherein the
2~ fibers are first inserted into a capillary tube where the
ends may overlap. The capillary tube is formed of a glass
having a refractive index lower than that of the fiber
cladding material. Heat is applied to the capillary tube
in the vicinity of the fiber overlap and the tube is
stretched until the diameter thereof approximates that of
the original fibers. The original cores of the pulled out
part become vanishingly small, their stretched diameters
being only about 1/lO0 the oriqinal diameters; the cladding
of the original fibers becomes the core of the coupling
section. Such a long thin coupler is very cumbersome and
fragile. Furthermore, such a coupler ~s lossy since the
original cladding takes the place of the vanished cores.
- 3 - t335870
In the region of the coupler where the flber core8 taper
from thelr "van~shingly small~ slze to their full slze, an
insufficient amount of power is transferred from the
cladding back to the core. Moreover, it is difficult to
maintain the cores straight and parallel to one another
when fibers are inserted into a tuke which is then
stretched unless specific steps are ta~en to position the
fibers. Such non-linear coupler cores can lead to
decreased coupling efficiency in single-mode couplers.
Japanese published application 60-140208 teaches a
coupler formed by pre-twisting a pair of fibers, inserting
them into a ~uartz tube, and heating and drawing the
central part of the tube to reduce its diameter. Resin is
then applied to the ends of the tube to seal the fibers
lS thereto. This coupler has the follcwing disadvantages.
During the collapse of the tube onto the fibers, the
capillary tube is not evacuated and the fibers are not
held ta~t. The fibers therefore meander in the tube,
thereby preventing the achievement of a predetermined
coupling when the tube is elongated by a predetermined
length. This als^ makes it diffic~ t to achieve low
coupler loss. The fibers are pre-twisted in order to
provide a sufficient length of fiber-to-fiber contact to
provide adequate coupling. Such a coupler cannot maintain
2S the polarization of an input optical signal; moreover, it
is difficult to make wavelength division multiplexed
couplers with twisted fibers.
Summary of the Invention
It is, therefore, an object of the present invention
to provide a method that overcomes the disadvantages of the
prior art. Another object is to provide a method of making
low cost, high optical guality optical couplers. A further
object is to provide a method of making optical couplers
that are capable of withstanding environmental abuses such
as temperature changes and mechanical influences and yet
effect a reliable and predictable transfer of energy
4 1 335870
between adjacent fibers. Yet another object is to provide a low cost,
efficient optical fiber coupler to which fibers may be easily
connection in the field as well as in the shop. A further object is
to provide an automated method of making optical couplers, the optical
properties of which closely conform to predetermined values. Yet
another object is to provide a mechanically strong, inexpensive
optical coupler that is capable of effecting a reliable and
predictable transfer of energy between adjacent fibers.
In accordance with a first embodiment of the method of the
present invention, there is provided a glass tube having first and
second opposite end portions and a midregion, a longitudinal
aperture extending from a first end of the tube to a second end
thereof. At least a portion of each of a plurality of glass glass
optical fibers are disposed within the longitudinal aperture. Each
fiber has a core and cladding. At least that part of each fiber
that is located in the tube midregion has no coating thereon. The
fibers have a coextensive relationship in the tube midregion, at
least a portion of at least one of the fibers in the midregion
extending beyond the first end of the tube, and at lea~t a portion
of at least another of the fibers in the midregion extending beyond
the second end of the tube. The aperture is evacuated, thereby
creating a differential pressure across the wall of the tube. The
tube midregion is heated, the combined effect of the pressure
differential and the heating causing the midregion to collapse onto
the fibers. The central portion of the midregion is drawn to
reduce the diameter thereof, the drawing step reducing the distance
between the cores of the fibers so that optical signals can be
coupled from one of the cores to another of the cores.
In accordance with another embodiment a fiber coupler is made
by disposing at least two glass optical fibers within the
longitudinal aperture of the tube, the fibers extending beyond both
ends of the tube. The fibers have a coextensive relationship in
the tube midregion. The aperture is evacuated and the tube
midregion is heated to cause the midregion to collapse onto the
fibers, as in the first embodiment, and at least a portion of the
midregion is drawn so that optical signals can be coupled from one
of the cores to another of the cores.
1 335870
In accordance with a further embodiment a fiber coupler is
made by disposing at least two glass glass optical fibers within
the longitudinal aperture of the tube, the fibers extending beyond
each of the ends of the tube. A differential pressure is created
across the wall of the tube, and the tube midregion is heated to
cause the midregion to collapse onto the fibers. - The central
portion of the midregion is drawn to reduce the diameter thereof,
the drawing step reducing the distance between the cores of the
fibers so that optical signals can be coupled from one of the cores
to another of the cores, whereby a drawn down portion is formed.
If the fibers have a coating thereon, the method further
comprises the step of removing a portion the coating intermediate
the ends thereof, cleansing the uncoated portions of the fibers,
and disposing the uncoated portions of the glass fibers within the
longitudinal aperture of the tube.
When the fibers are disposed in the tube, it is sometimes
desirable to effect a tension therein by holding the fibers taut.
This can be done by affixing the fibers to the first end portion of
the tube, pulling that portion of the fibers that extend from the
the aperture at the second end of the tube to apply a tension
thereto, and affixing the fibers to the second end portion of the
tube.
The step of affixing the fibers to the end portions of the
tube may comprise applying glue to the fibers to seal the fibers to
the end portions. The glue can be applied to less than the entire
circumfirential region around the fibers, thereby leaving an
opening between the aperture and the glue to retain an access to
the aperture at that end of the tube through which the aperture can
be evacuated. The end portions of the glass tube may be provided
with enlarged tapered apertures for providing access to the
longitudinal aperture from the ends of the tube. The tapered
apertures facilitate the gluing of the fibers to the end portions
without eliminating access to the longitudinal aperture.
- After the coupler is drawn, additional glue can be inserted
into the access opening between the aperture and the glue, whereby
the access to the aperture is sealed. A sufficient amount of glue
can be employed that some glue enters the aperture and contacts the
stripped portions of the fibers.
5a 1 335870
The step of evacuating may comprise disposing a hollow glass
filament contiguous to any of the fi~ers that extend from that end
of the tube, the filament extending into the longitudinal aperture.
Glue is applied to the fibers and filament to seal them to the
first end portion of the tube. The aperture is evacuated by
applying a vacuum to the hollow glass filament. After the coupler
is drawn, that portion of the hollow glass filament extending
beyond the glue can be removed, and that end of the filament from
which the portion has been removed can be sealed.
The drawing step can be performed such that the range of
ratios of the starting diameter of the tube to the diameter of the
drawn section of the midregion is between approximately 1/2 and
1/20. The drawing step can comprise allowing the tube to cool,
reheating the central portion of the ~idregion, and stretching the
central portion. The reheating step can be of shorter time
duration than the heating step. The maximum temperature to which
the tube is heated during the reheating step can be less than the
temperature to which the tube is heated during the heating step.
Thus in another embodiment as claimed in this divisional
application the invention provides an optical fiber coupler comprising:
an elongated glass body having a solid midregion and first and second opposite
end portions which respectively extend frorn said midregion to the first and second ends
of said body,
first and second longitudinal apertures respectively extending from said first and
second ends of said body to the midregion thereof, said apertures being tapered so that
they are larger at said first and second ends than they are within said body,
at least two glass optical fibers, each having a core and cladding, extending
through said body, at least one of said fibers extending beyond the first and second ends
of said body,
first bonding means within said first aperture for attaching the fibers therein to
said first end portion, said first bonding means sealing less than the entirety of said first
aperture thereby leaving an access to said first aperture,
first sealing means for closing the access to said first aperture,
second bonding means within said second aperture for attaching bonding means
sealing less than the entirety of said second aperture thereby leaving an access to said
second aperture,
second sealing means for closing the access to said second aperture.
5b 1 3 3 5 8 7 0
In a further embodiment this application also provides an optical
fiber coupler comprising:
an elongated glass body having a solid midregion and first and second opposite
end portions which respectively extend from said midregion to the first and second ends
of said body,
first and second longitudinal apertures respectively extending from said first and
second ends of said body to the midregion thereof, said apertures being tapered so that
they are larger at said first and second ends than they are within said body,
at least two optical glass fibers, each having a core and cladding, extending
through said body, at least one of said fibers extending beyond the first and second ends
of said body,
first bonding means within said first aperture for attaching the fibers therein to
said first end portion,
a hollow glass filament having first and second ends, said filament extending
through said first bonding means so that the second end thereof extends into said first
aperture,
first sealing means covering the first end of said filament, second bonding means
within said second aperture for attaching the fibers therein to said second end portion.
- 6 - 1 335~70
Br~ef ~escription of the Drawinq
Flg. 1 is a cross-sectional view of a glass tube
suitable for the purposes of the present invention.
S Fig. 2 is a cross-sectional view of the tube of Fig. 1
within which a pair of optical fibers are disposed.
Fig. 2a is a cross-sectional view of one end portion
of the tube of Fig. 2.
Fig. 3 is a cross-sectional view illustrating further
steps in the method of the present invention.
Fig. 4 is a cross-sectional view illustrating
additional steps in the process of the present invention.
Fig. 5 is a cross-sectional view illustrating the
collapse of the glass tube around the fib!ers to form a
solid midregion.
Fig. 6 is a cross-sectional view through the solid
midregion of Fig. 5 along lines 6-6.
Fig. 7 is a cross-sect_onal illustration cf the fiber
coupler of the present invention after it has been drawn
down and sealed at its ends.
Figs. 8 and 9 are cross-sectional views illustrating
additional methods of provicing access to the tube aperture
during processing.
Fig. 10 is a cross-sectional view illustrating an
additional method of providing access to the tube aperture,
and in addition, illustrates a method of evacuating the
tube.
Fig. 11 is a cross-sectional view taken along lines
11-11 of Fig. 10.
Fig. 12 is a schematic illustration of an apparatus
for inserting fibers into the tube.
Fig. 1~ is a schematic illustration of an apparatus
for collapsing the tube and draw_ng the midregion thereof.
` ~ 7 ~ 1 335870
Descr~ptlon of the Preferred Embodlments
The drawings are not intended to indicate scale or
relative proportions of the elements shown therein.
Referr~ng to Fig. 1, there is provided a hollow glass
cylindrical tube 10 having a longitudinal aperture or bore
12 provided along the longitudinal axis thereof. Tube 10
may comprise a capillary tube which may be formed as
hereinafter described in detail or as taught in my
copending application entitled "Capillary Splice and
Method", S.N. 082,680 (Berkey 9), filed on Auqust 7 1987
now USP 4,822,389~
Tapered apertures 14 and 16 form funnel-like entrances to
longitudinal aperture 12 at end surfaces 18 and 20,
respectively. The tapered apertures facilitate the
insertion of fibers into aperture 12, since the maximum
cross-sectional dimension thereof may be less tha~. 400 ~m.
Tr.e soften.r.g point temperature of tube 10 should be
lower than that of the fibers that are to be inserted
therein. Suitable tube compositions are SiO2 doped with 1
to 25 wt. ~ B2O3 and SiO2 d~ped with 0.1 to approximately
2.5 wt. % fluorine. ~ preferred composition is
borosilicate glass comprising SiO2 doped with 8-10 wt. %
B2O3. In addition to lowering the softening point
temperature of SiO2, B2O3 and F also advantageously
decrease the refractive index thereof.
Referring to Fig. 2, a pair of optical fibers 22 and
24, each having a core, cladding, and protective coating,
extend through longitudinal aperture 12, a sufficient
length of each fiber extending beyond each end of tube 10
to make connection thereto, a length of 1 meter having been
found to be sufficient. A portion of ~he coating
intermediate the ends of each fiber is remc~ed for a
distance slightly shorter than the length of aperture 12.
The fibers are wiped to eliminate residual material. The
uncoated portions of the fibers are disposed intermediate
end surfaces 18 and 20 of hollow member 10. Preferably,
- 8 - I 3358 7(~
the uncoated portions of fibers 22 and 2~ are
longitudlnally centered with~n aperture 12. For
convenience and ease of illustration, fibers 22 and 24 are
drawn in the figures with a light line within most of
aperture 12 and a heavy line from within the ends of
aperture 12 to the exterior of tube 10. The light lines
represent uncoated portions of the fibers while the heavy
lines represent coated portions thereof. Fig. 2a is an
enlarged cross-sectional view of one end of Fig. 2 showing
coatings 23 and 25 on optical fibers 22 and 24
respectively.
The fibers may be maintained parallel to one another
within aperture 12 or may be twisted any amount including
180 or more as illustrated in Fig. 3. Twisting the fibers
is a well known technique that has been used to maintain
the fibers in mutual contact during the step of fusing the
fibers together. If vacuum is employed during collapse of
tube 10 as herein described, twisting of the fibers is not
critical since the vacuum will assist collapsing the tube
and maintaining the fibers in longitudinal contact with
each other. It is noted tha~ for certain types of ~oupling
~evices, such as w~M coup;ers and polarization retaining
couplers, the fibers must be kept untwisted and must be
maintained parallel to one another.
The assembly comprising tube 10 and the fibers
extending therethrough is preferably subjected to a final
cleaning step prior to collapsing tube 10 and fusing
together the stripped fiber portions. This step is
important since small pieces of coating material or other
contaminants -may remain on the uncoated portions of the
fibers after they have been inserted into tube 10. The
cleaning step may comprise flowing a cleaning fluid through
aperture 12 and over the stripped portions of fibers 22 and
24. The cleaning fluid may comprise a liquid cleaning
solution such as a 30 % ammonia solution or a gas such as
air. Furthermore, the fibers are preferably held taut
during the tube collapse step. A variety of techniques can
9 ~ 335~`70
be employed to effect the steps of cleaning the aperture
and tenslon~ng the fibers; preferred techniques belng
described in the following specific embodiments.
In a first embodimRnt, a pair of optical fibers 22 and
24, each havlng a core, cladding, and protectlve coatinq,
are suitably prepared by removing a portion of the coating
intermRdiate the ends thereof as descr~bed above. The
uncoated section is cleaned by wiping with a lintless cloth
to remove residual material. Fibers 22 and 24 are fed
through longitudinal aperture 12 so that a suitable length
extends beyond each end of tube 10 for connection purposes.
The uncoated portions of the fibers are disposed
intermediate end surfaces 18 and 20 of hollow member 10 as
shown in Fig. 2, the uncoated portion of fibers 22 and 24
preferably being centered within aperture 12.
For certain types of couplers, fibers 22 and 24 may
then be twisted within longitudinal aperture 12 about 180
as illustrated in Fig. 3. A hollow glass fila~ent 26 is
inserted into the end portion of tube 10 so that it extends
a short distance into longitudinal aperture 12. Fibers 22
and 24 and filament 26 are then secured to the end portion
of member 10 by applying a quantity of glue 30 within and
about tapered aperture 14. The process is then repeated at
the other end of member lO by inserting a second hollow
glass filament 28 into longitudinal aperture 12 and
applyinq a quantity of glue 32 to the fibers within and
about aperture 16. While qlue 32 is setting or curing, a
slight tension is applied to fibers 22 and 24. Glue 30 and
32 may consist of any bonding material such as cement,
adhesive or the like, W cura~le epoxy being preferred.
The assembly so formed is then placed in a suitable
mounting device or holder 34, such as a tinners clamp.
Hollow filament 28 may then be connected to a suitable
source of vacuum (not shown) illustrated by arrow 36.
Alternatively, a tube connected to a source of vacuum may
be placed around the end of capillary tube 10 so that
hollow filament 28 and fibers 22 and 24 extend into the
- lo - 1 3 3 5 8 7 0
evacuated tube. If hollow fl~ament 26 is lnserted into a
liquid cleaning flu~d, the fluid is drawn through
longitudinal aperture 12 by the vacuum applied to hollow
filament 28 whereby it cleans the interior of longitudinal
S aperture 12 and those portions of fibers 22 and 2~ and
hollow filaments 26 and 28 that are disposed therein.
If a liguid cleaning fluid is employed, midregion 38
of the assembly so formed is then heated by a suitable heat
source 40 as illustrated in Fig. 4, to vaporize the liquid
and dry out the assembly. This drying step is not needed
if a gas is used as the cleaning fluid.
In accordance with one embodiment of the present
invention, tube 10 is heated and collapsed onto fibers 22
and 24, and thereafter, the midregion of tube 10 is heated
and stretched to bring the fiber cores closer together
along a distance sufficient to accomplish a predetermined
type of coupling. This is accomplished by first heating
midregion 38 to the softening point of the borosiiicate
glass tube 10 by means of heat source 40, which may
comprise an oxygen-hydrogen burner, a gas-oxygen burner, or
the like. Burner 40 may remain stationary or it may
traverse midregion 38 in the direction toward vacuum source
36 as shown by arrow 41 in Fig. 4. It is an optional
feature of the tube collapse step to apply a vacuum source
to both hollow filaments 26 and 28, in which case the
direction of burner traverse is immaterial. The step of
subjecting midregion 38 to heat source 40 causes the
material of tube 10 at midregion 38 to collapse about
fibers 22 and 24 as additionally illustrated in Fig. 5.
Fig. 6 illustrates the collapsed midregion 38 of tube lO
about fibers 22 and 24 along line 6-6 of Fig. 5. The
portion described as midregion 38 becomes a solid region
that is preferably free of air lines, bubbles, or the like.
The assembly so formed is removed from holder 34 and
placed in a precision gl~ss working lathe illustrated by
members 42 and 44 in Fig. 5. The solid midregion 38 is
then subjected to the flame from an oxygen-hydrogen burner
~`~
- 11 1 3 3 g 8 7 0
46 until a portion of the solid midregion 38 ~s heated to
the softening point thereof. If the entire midreg~on were
stretched, the end portion of the light coupling region of
the fibers could be exposed to the aperture. Stretching
only the central portion of the collapse~ midregion ensures
that the coupling region of the fibers will be embedded in
the matrix glass of the capillary tube. The flame is
removed and the softened portion of midregion 38 is pulled
or drawn down by action of the glass working lathe to
reduce the diameter thereof as illustrated by region 48 of
Fig. 7. The diameter of drawn down region 48 will vary as
a function of various fiber and operational parameters.
The ratio of the drawn down dia~eter of region 48 to the
startinq diameter of midregion 38 ~the draw down ratio) is
determined by the optical characteristics of the particular
device being made. It is well known that such draw down
ratios are a function of t~.e ratio of the signal split
~etween the fibers, the refra_tive index difference between
the tube and the fiber cladding, the outside diameter of
~he fiber cladding, the diameter of the fiber core, signal
operating wavelength, cutoff wavelength, the tolerable
excess loss, and the like. A preferred range of draw down
ratios is between about 1/2 to 1/20; however, couplers can
be made having draw down ratios outside this range.
2j As illustrated in Fig. 5, the portion of member 10
held by glass working lathe member 42 is held stationary
while t~e portion of member lO held by lathe member 44 is
traversed in the direction of arrow 50 to obtain drawn down
region 48. In practice, such a pull down or draw down
takes approximately 1/2 second. Alternative drawing
techniques involve the movement of lathe mer~er 42 in the
same direction as that in which member 44 moves or in a
direction opposite that in w~.ich member 44 mo-es.
The assembly would not need to be rotated if the draw
down portion of midregion 38 were heated by a ring burner
which would uniformly heat that region around its
periphery. The draw down method would otherwise be the
(~ !~
- 12 - 1 335870
same. In the embod~ment wherein a ring burner i~ employed,
the step of collapsing tube 10 onto fibers 22 and 2~ and
the step of forming drawn down region 48 may be performed
on the same apparatus. If the collapse and stretch
operations are performed in the same apparatus, it is
preferred that tube 10 be allowed to cool prior to being
reheated for the stretch step. This temporal separat~on of
the two steps results in better process control and
therefore better reproducibility. Furthermore, tube 10 can
be disposed in any orientation including vertical and
horizontal during the tube collapse and/or drawing
operations.
After the draw down, the exposed ends of hollow
filaments 26 and 28 are removed by breaking them off at the
surface of glue 30 and 32, and the apertured ends thereof
are sealed with quantities 54 and 56 of glue as heretofore
~escribed. The resulting assembly comprises fiber optic
c~up'er 52 of Fig. 7. The coup;er can be further processed
by packaging, not shown, for additional stiffness if
desired. Coupler 52 functions to couple a signal in
optical fiber 22 to optical fiber 24 and vice versa.
In accordance with the above-described embodiment, the
s.eps of collapsing and stretching are separately
performed. This is advantageous since more control can be
exerted over each step if the tube is allowed to cool prior
to heating it for the stretching operation. A central
portion of the solid collapsed midregion can be stretched,
thereby ~eeping the stretched portions of the optical
fibers completely enclosed in the matrix glass of the tube.
This improved hermeticity is advantageous since it prevents
the stretched portions of the fibers from being adversely
affected by water and the like, a factor that can adversely
m~dify the optical characteristics of the coupler.
Low loss couplers have also been made by an
alternative embodiment wherein the steps of collapsing the
tube onto the fibers and drawing or stretching the
midregion of the tube are performed in a single heating
- - 13 - 1 3 3 5 8 7 '~
operation. In accordance with this modifled embodLment,
the fibers are inserted into the capillary tube and are
glued taut to the ends thereof such that there are access
openings to the aperture. This assembly is placed in a
precision glass working lathe as described above. A flame
is applied to a small portion of the midregion until the
softening point of the materials is reached, and the ~eated
section is stretched. For a given amount of coupling, the
amount of tube elongation is greater in this embodiment
than in that embodiment wherein the tube collapse and the
medregion stretching steps are separately performed.
Finally, glue is applied to the ends of the device to close
t~.e openings to the aperture.
The disadvantages of this embodiment are a reduction
in hermeticity and an adverse affect on manufacturing
reproducibility, i.e. stretching to a predetermined length
does not always result in the desired coupling
c~ara~teristics. However, this embodiment has some
advanta~es over other methods. The method is simpler in
that it can be performed without vacuum and the separate
tube collapse step is eliminated. Low loss couplers have
been formed by this method, device losses as low as 0.05 dB
1300 ~m having been measured.
In another alternative embodiment a hollow fiber is
employed in only one end of tube lO. Such an embodiment is
similar to that resulting in the formation of coupler 52
except that the internal cleansing step described will not
be practical.
The hollow fibers are eliminated entirely in the
embcdiments of Figs. 8 through 10, wherein elements similar
to those of Fig. 2 are represented by primed reference
numerals.
The initial steps needed to form the embodiment of
Fig. 8 are the same as those employed to form that of Fig.
3. A filament similar to filament 28, which may be hollow
or solid, is inserted through tapered aperture 16' so that
it protrudes a short distance into aperture 12' as shown in
- 14 _ l 3 3 5 8 7 0
Fig. 3. After the uncoated portions of fibers 22 ' and 24 '
have been centered in aperture 12 ', a quantity of glue 58 is
applied within and about tapered aperture 16 ' . As shown in
this figure, the glue advantageously extends into aperture 12'
5 a sufficient distance to contact the glass cladding of fibers
22 ' and 24 ' . As glue 58 begins to set or cure, its viscosity
becomes sufficiently high so that removal of the extra
filament will leave an aperture 59 into which bonding material
58 cannot flow and close. An aperture similar to aperture 59
could also be formed in the glue located at the opposite end
of tube 10'. To clean and/or evacuate aperture 12 ', a vacuum
attachment tube may be placed around the periphery of tube 10'
at end surface 20 ' thereof as described above.
In the embodiment of Fig. 9 glue 61 completely seals
15 tapered aperture 16 ' . A radial bore 62 near the end 20 ' of
tube 10' provides access to aperture 12 ' for cleaning/or
evacuating purposes. Aperture 12 ' can be evacuated through
bore 62 by attaching an annular vacuum fixture 64 to tube 10'
so that bore 62 opens into annular slot 63. Arrow V indicates
20 that a vacuum source is connected to fixture 64. A radial
bore similar to bore 62 can be formed at the opposite end of
tube 10' for purposes such as supplying cleaning fluid or gas
to aperture 12 ' and/or evacuating aperture 12 ' .
For a coupler manufacturing process to consistently
25 produce couplers havingpredeterminedoptical characteristics,
all of the process steps, including the step of inserting the
fibers into the capillary tube should be uniformly performed
on each coupler made. Discussed along with a description of
the method of forming the embodiment of Figs. 10 and 11 is a
preferred fiber insertion method which enhances process
uniformity. It is advantageous to employ a fiber insertion
station which meets the following criterion. The mechanism
which is to hold the fibers should be properly aligned since
the fibers are preferably kept untwisted and straight. Means
3 5 should
- 15 - l 335870
be provided for holding the fibers under a sl1ght tenslon
during the gluinq step to el~mlnate the occurrence of fiber
slack or sag during further processing steps, especially
during the step of colla~sing the capillary tube onto the
fibers. The appearance of slack ~n one or both of the
fibers coult cause the resultant device to exhib~t an
excessive loss. The area around the station should be free
from excessive dust and other particulates that could be
drawn into the capillary tube and lodged inside, since
seeds could result from such particulate matter during the
collapse and redraw steps. The excessive attenuation that
can result from such seeds could render the coupler
useless.
A suitable fiber insertion station, which is shown in
Fig. 12, comprises aligned blocks 67, 74, 76, 79, 82 and
83. Rubber surfaced clamps 70 and 71 are capable of
retaining optical fibers against block 67. Similar clamps
84 and 8j are associated with block 83. The clamps, which
are spring biased against the blocks, can be withdrawn from
contact with the blocks by depressing a handle connected
thereto. ~lock 74 cortains spaced grco~es ~2 and 73 that
are aligned with grooves 80 and 81 of b'oc~ 82. A single
groove 75 in the surface_of block 76 is aligned with
similar groove 78 of block 79. She illustrated grooves may
be U-shaped and may have a width that is just sufficient to
slidingly accomodate the fiber or fibers that are s~tuated
therein.
Two lengths 22'and 24'of coated optical fiber are
severed from a reel of fiber. An end of each of fibers
22'and 24' is secured by clamps 70 and 71, respectively.
The entire lengths of the fibers are wiped with a lintless
cloth dampened with a suitable cleaning solution such as
ethyl alcohol.
There is selected a capillary tube 10', the aperture
of which is preferably just large enough to accept the
coated portions of the optical fibers. Such a relationship
between the coated fibers and the aperture prevents the
_ ~ 1 33~870
- 16 -
ends of the fibers from twisting within the tube. As
illustrated in Fig. 11, certain hole cross-sectional shapes
such as diamond, square and the like facilitate the proper
alignment of the fibers in the tube. The aperture diameter
should not be so small that it is difficult to thread the
fibers therethrough, since this could cause the coating to
smear on the inside of the tube. The smeared region of the
tube could cause the resultant coupler to contain seeds that
would degrade the coupler's performance. In order to
facilitate easy movement of the tube along the fibers, a small
amount of ethyl alcohol may be squirted into the tube. This
functions as a temporary lubricant which will readily
evaporate. The capillary tube is threaded onto the fibers and
moved to approximately the position shown adjacent block 76.
The fibers are pulled slightly so that they are under some
tension and the remaining ends thereof are then restrained by
clamps 84 and 85. A mechanical stripping tool is utilized to
remove a portion of the coating from each fiber at a location
thereon between tube 10' and block 79. The length of the
stripped section of fiber is slightly shorter than the length
of the capillary tube aperture to allow the coating to extend
into both ends thereof, thereby properly positioning the
fibers within the aperture cross-section. The lengths of the
stripped regions should be about equal, and those regions
should be adjacent one another.
Using a dampened lintless cloth, the two fibers are
grasped at the left end of tube 10' and are wiped firmly, the
cloth being moved away from the tube and across the stripped
regions. This step removes any loose material generated by
the coating stripping step and leaves a pristine surface on
the stripped regions of the fibers. The fibers are then
placed into grooves 75 and 78 which help to hold the fibers
straight and adjacent one another. Clamp 84 is released and
then reclamped after fiber 22~ has been retensioned; fiber 24'
is then similarly retensioned.
- 17 - 1 3 3 5 8 7 0
The cap1llary tube ~s moved toward block 79 and
positioned such that it ~s centered over the stripped
region as shown in Fig. 10. A small amount 87 of glue is
applied to one side of fibers 22' and 2~' to attach them to
S one side of tapered aperture 16' while leaving an opening
88 which penmits access to longitudinal aperture 12'
between glue 87 and the remainder of the tapered aperture
16'. A drop 89 of glue is similarly applied between the
fibers and tapered aperture 14', leaving aperture access
opening 90 between glue 89 and tapered aperture 14'.
Depending upon the size of the capillary tube aperture, it
can be difficult or even impossible to glue the fibers to
the tube end portions without blocking the aperture unless
the tube is provided with tapered apertures 14' and 16'.
1~ Openings 88 and 89 permit the flow of fluid through
aperture 12' during the final wash, and also permit the
evacuation o' aperture 12' during the collapse of tube 10'.
If the glue s a W light curable epoxy, W lisht source 86
is directed on the first applied drop of epoxy to cure it
before the second drop is applied to the remaining end.
A'ter the se:~nd drop ~s a~?lied, source ~6 is moved as
indicated by ~he arrows and directed onto the second drop.
The pigtails or sections of fiber extending from the
ends of tube 10' can be color coded. At this ti~e the
2; fibers within the capillary tu~e are visually checked for
internal twists. A twist of more than 180- can be seen by
the naked eye. Also, a laser beam can be launched into
that end of fiber 22' protruding from clamp 84. If there
is no twist present, the light emanates from that end of
fiber 22' protruding from clamp 70. An orientation mark
can be placed on the upper surface of tube 10' so that the
fibers can be oriented in the same manner with respect to
the draw apparatus for each coupler that is made, thereby
ensuring every coupler preform is subjected to uniform
process conditions.
A preferred apparatus for performing the tube
collapsing and stretching steps is shown in Fig. 13.
18 - l 335~7
Chucks 92 and 93, whlch are used to secure tbe coupler
prefonm ln this apparatus, are mounted on motor controlled
stages, which are preferably controlled by a computer. The
numerals 92 and 93 are also used to designate the stages.
S Symmetry is an important reguirement for the collapse and
stretch steps; therefore, chucks 92 and 93 must be aligned
to prevent the occurrence in the coupler of an offset which
can adversely affect device loss and which can also
adversely affect coupler bidirectionality, that
characteristic whereby coupler output performance is
substantially uniform regardless of which end of a fiber is
selected as the input port. Coupler bidirectionality is
also enhanced by locating the burner centrally along the
coupler preform so that it heats the prefor~ evenly. A
symmetrically designed burner such as ring burner 94 is
suitable for evenly heating the capillary tube midregion.
Heat shield 95 protects the apparatus located above the
burner.
Coupler preform 91 of Fig. 10 is inserted through ring
burner 94 with the orientation mark facing a predetenmined
direction. The preform is clamped to the draw chucks, and
vacuum attachments 96 and 101 are attached to the ends
thereof. Vacuum attachment 96, which is shown in
cross-section in Fig. 10, may comprise a short, somewhat
rigid section of rubber tube havinq a vacuum line 97
extending radially therefro~. One end of a length of thin
rubber tubing 98 is attached to that end of vacuum
attachment 96 that is opposite preform 91; the remaining
end of the tubing extends between clamp jaws 99. Upper
vacuum attachment 101 is similarly associated with line
102, tubing 103 and clamp jaws 104. Fibers 22' and 24'
extend from tubing 98 and 103.
Vacuum is applied to the lower portion of couple~
preform 91 for a time sufficient to wash aperture 12' by
clamping jaws 99 on tubing 98. The upper line is vented to
air during this time by leaving clamp jaws 104 open. This
"air wash" puIls from aperture 12' any loose debris which
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1 335870
has accumulated thereln dur~nq the fiber insert~on step.
Jaws 104 are then clamped against tub~ng 103 to apply
vacuum to the upper portion of preform 91.
The cap~llary tube collapse step entails heating the
coupler preform with the flame from ring burner 9~ for a
short period of tLme, typically 25 seconds, to increase the
temperature of the midregion of the tube to the softening
temperature. With the assistance of the differential
pressure on the tube, this causes the ~atrix qlass to
collapse onto the fibers and urges them into mutual
contact. The tube matrix glass surrounds the fibers and
fills the aperture to form a solid structure, thereby
eliminating dirlines in the coupling region. The aperture
is preferably evacuated from both ends thereof during the
lS collapse step. The longitudinal length of the res on that
is to be collapsed is determined by the flame temperature,
as determined by the flow of gases to the burner, and the
time duration of the flame.
The central portion of the collapsed midregion of the
tube can be stretched without removing the device from the
a?Far~tus in whic~. the tube was collapsed. After -..e tube
cools, the flame is reignited, and the center of the
collapsed region is reheated. The flame duration for the
stretch process, which depends upon the desired coupler
characteristics, is usually between 10 and 20 seconds. The
shorter heating period for the stretch step results in a
stretched region that is shorter than the collapsed region.
After the collapsed tube is reheated, stages 92 and 93 pull
in opposite directions until the coupler length has been
increased by a predetermined amount. If properly aligned
apparatus is employed and if the process parameters are
carefully controlled, all couplers formed by the process
will possess similar optical characteristics.
The amount of stretching to which the capillary tube
must be subjected to achieve a given type of coupler is
initially determined by injecting light energy into one
input fiber of a collapsed coupler preform and monitoring
- 20 - 1 3 3 5 ~ 7 0
operation. To accompllsh thls purpose, one of the f~ber
pigtails is aligned with a light source, and both pigtails
at the other end of the device are coupled to light
detectors. The predetermine~ ratio of the dynam~c output
powers can be used as an interrupt to cause stages 92 and
93 to stop pulling the sample. After havlng determlned the
proper stretching distance to achieve predetermined
coupling characteristics, the apparatus can be programmed
to move the stages that proper stretching distance during
the fabrication of subse~uent couplers that are to have
said predetermined characteristics.
It is conventional practice to monitor output signals
to control process steps in the manufacture of optical
devices as evidenced by U.S. patents ~os. 4,392,712 and
4,726,643, U.~. Patent Application No. GB 2,183,866 A and
International Publication ~o. WO 84/04822. Furthermore,
computers are often employed to in feedback systems which
automatically perform such monitor and control functions.
A suitably programmed PDP 11-73 micro-computer can be
utilized to perform these functicns. The timing sequences
that have been used in the fabrication of a partic~lar ;ype
of coupler can be entered in a separate multiple command
file that the computer recalls at run-time. The collapse
and s~retch steps that are required to make that particular
coupler can be executed in succession by the co~puter on
each coupler preform to reproducibly manufacture couplers.
The process parameters that can be controlled by the
computer to ensure coupler reproducibility are heating
times and temperatures, flow rates of gases, and the rate
at which the stages pull and stretch the coupler preform.
~eproducibility is also a function of the resoiution of
stages 92 and 93.
After the coupler has cooled, the vacuum lines are
removed from the coupIer and a drop of glue is applied to
each end of the capillary tube where it flows at least
partially into the longitudinal aperture. This produces a
hermetic seal and also increases the pull strength of the
1 335870
- 21 -
devices. The coupler is then removed from the draw and is
ready to be packaged.
Although the foregoing description has been related to
couplers made from pairs of optical fibers, it will be
evident that the invention is also applicable to couplers
made from more than two fibers.
The following ~pecific examples ut~lize glass
capillary tubes formed in the manner described in my
co-pending application S.N. 082,679, filed August 7, 1987.*
Glass particulate material was applied to a cylindrical
mandrel, consolidated, drawn, a~d dried in accordance with
the teachings of U.S. Pate~ts Numbers Re. 28,029,
3,884,550, 4,125,388 and 4,286,978
More
1~ specifically, the particulate material was deposited on a
mandrel to form a porous, cylindrically-shaped preform.
The mandrel was removed an~ the porous preform was
consolidated to form a tubular glass body which was heated
and redrawn to an outside dia-_ter o about 2.8 to 3 mm.
One end of the resultant capil~ary tube was attached to a
source of air pressure, and wh~le the tube was rotated, a
flame was di.ected onto th~ tu~e zt spaced intervals. The
air pressure within the tube caused a bubble to be formed
at each region of the tube softened by the flame. The tube
was scored at the center of each bubble and then severed at
each score line to produce a capillary tube having tapered
apertures at each end thereof. My copending patent
application S.N. 082,679 filed August 7, 1987 (Berkey 12) *
teaches a method of producing apertures of non-circular
cross-section by shrinking the tube onto a carbon mandrel
of desired cross-section and then burning ou~ the mandrel.
Example l
~ _ Reference will be made to Figs. 1-7 during the
description of this example. Capillary tube 10 was formed
as described above; it had an outside diameter of about 2.8
mm, a longitudinal aperture diameter of 400 ~m, and had a
length of S.1 cm. Tube 10 was formed of a borosilicate
glass containing 8 wt % B O . Two single-mode optical
* now USP 4,750,926
1 335870
`_ ~
- 22 -
fibers having an outside diameter of 125 ~m were each cut to
a length of approximately 2 m. Each fiber comprised a core,
a cladding and a urethane acrylate resin coating. A
commercially available mechanical stripper was used to remove
the resin coating from approximately 3.8 cm (1 1/2 inches) of
the central portion of each fiber.
The uncoated portions of the fibers were wiped with a
lintless cloth to remove residual matter, and the fibers were
pulled through longitudinal aperture 12 until the uncoated
portions of the fibers were approximately centered therein.
A hollow glass fiber 28 having an outside diameter of
approximately 125 ~m was inserted approximately 0.3 to 0.6 cm
(1/8 to 1/4 inch) into end 20 of the tube. A quantity of
Norland W curable glue was disposed within tapered aperture
16 about the three fibers and cured by exposure to W light
for about 1 minute. In this manner the optical fibers and
filament 28 were rigidly affixed to the end of tube 10.
The two optical fibers were then twisted 180 within
aperture 12, and a second hollow fiber 26 was inserted
approximately 0.3 to 0.6 cm into the other end of longitudinal
aperture 12. A slight tension was applied to the two fibers
and a drop of W curable glue was applied to the fibers within
tapered aperture 14. The glue was cured as described above.
The assembly so formed was mounted in a tinner's clamp that
was modified by cutting away the central portion and one end
portion of the clamping region such that when the coupler
assembly was mounted, midregion 38 and one end surface 18 was
exposed. A tube connected to a vacuum source was connected
to one end of the capillary tube such that the optical fibers
and hollow filament were disposed inside the evacuated tube.
In this manner, longitudinal aperture 12 was evacuated through
hollow filament 28. Hollow fiber 26 was inserted into a
beaker of 30% ammonia solution. The ammonia solution was
sucked into aperture 12 whereby the aperture and the outside
surfaces of the optical fibers were cleansed for approximately
~,.
1 335870
- 23 -
10 seconds. Hollow fiber 26 was then removed from the beaker
of cleansing solution. After as much of the liquid as
possible was removed form aperture 12 by the vacuum source,
a burner was directed at tube 10 for about 20 seconds to
assist in drying out the interior thereof.
The midregion 38 of tube 10 was then heated to the
softening point of the borosilicate glass by an oxygen-
hydrogen burner whereupon the glass collapsed around the
optical fibers within the longitudinal aperture. The flame
was then traversed through the midregion in the direction of
the vacuum source so that as the material of the tube
collapsed about the optical fibers; residual matter within the
longitudinal aperture being sucked out by the vacuum. In this
manner a solid midregion was formed free of air lines or
bubbles.
The assembly so formed was then removed from the modified
tinner's clamp and placed in a precision glass working lathe.
The lathe was a Heathway glass working lathe having a computer
controlled pull down or drawn down mechanism. The flame from
an oxygen-hydrogen gas burner was then applied to a small
portion of the solid midregion until the softening point of
the materials was reached, whereupon the computer controlled
pull down apparatus stretched the heated section for an
interval of approximately 0.5 second. The diameter of the
pulled down section was approximately 0.7mm.
Thereafter, hollow filaments 26 and 28 were broken off,
and W curable glue was applied to the ends of the device to
cover the resultant holes. The assembly was then packaged
within a stainless steel tube for stiffness. Signal losses
measured on the coupler so formed were typically in the 0.05
to 0.7 dB range at 1300 ~m wavelength. This produced a 50:50
signal split in the fibers having a 1200 ~m cutoff wavelength.
Example 2
There was provided a capillary tube 10 of the type
described in Example 1. Two single-mode optical fibers of
- 24 - l 335870
the type described in Example 1 were prepared in accordance
with that example. After the uncoated portions of the
fibers were wiped, they were pulled through longitudinal
aperture 12 and the uncoated portions were approximately
centered therein. A guantity of Norland W curable glue
was carefully placed between the fibers and one side of
tapered aperture 16, leaving a small opening to aperture
12. The glue was cured by exposure to W light for about 1
minute. In this manner the optical fibers were rigidly
affixed to the end of tube 10. After a slight tension was
applied to the two fibers, a drop of W curable glue was
carefully applied and cured, as described above, to rigidly
adhere the fibers to tapered aperture 14.
The assembly so formed was placed in the precision
l; glass working lathe of Example 1. ~he flame from an
oxygen-hydrogen gas burner was then applied to a small
portion of the midregion until the softening point of the
materials was reached, whereupon the computer controlled
plll down ap~aratus stretched the heated section for an
2~ interval of approximately 0.6 second. The amount of
elongation of the capillary tube was about 4 cm, about
~-~ice the amount of tube elong2~ion needed in Exam.ple 1.
The diameter of the drawn down section was approximately
0.4 mm. W curable glue was applied to the ends of the
device to close the openings to the aperture.
Devices formed by this method functioned as 3 dB
couplers, that is, it produced a 50:50 signal split.
Signal losses measured on these devices were as low as 0.05
dB 1300 ~m.
Example 3
Employing the apparatus of Figs. 12 and 13, the
following steps performed in order to fabricate a
single-mode 3 dB coupler. Reference will also ~e made to
the coupler preform of Figs. 10 and ll. Two lengths 22'and
24' of coated single-mode optical fiber were severed from a
* Trade-mark
- 25 - l 3 3 5 8 7 0
reel of fiber. The optical fibers had a diameter of 125
~m, and the diameter of the coated fiber was 160 ~m. The
length of each piece of fiber was about 2 meters. The ends
of the fibers were secured by clamps 70 and 71, and the
fibers were wiped with a lintless cloth dampened with ethyl
alcohol.
Capillary tube 10' had an outside diameter of about
2.8 mm and a length of about 4.12 cm. The longitudinal
aperture was diamond-shaped, each side of the diamond
having a length of about 310 ~m. Tube 10' was formed of a
borosilicate giass containing 8 wt % B2O3. The minimum
cross-sectional dimension of the diamond-shaped aperture
was just large enough to accept the coated portions of the
optical fibers in the manner illustrated in Fig. 11. A
small amount of ethyl alcohol was squirted into the
capillary tube which was then threaded onto the fibers and
moved to approximately the position shown in Fig. 12. The
fibers were pulled slightly a~.d t~e remaining ends thereof
were clamped. A section of coating about 3.2 cm (1.25
inch) long was removed from each fiber at a location
thereon between tube 77 and block 79. The length of the
stripped ~ction of fiber w~s slightly shorter Ihan the
length of the capillary tube aperture. The two fibers were
again wiped with a lintless cloth that had been dampened
with ethyl alcohol to remove loose material generated by
the coating stripping step. The fibers were placed into
grooves 75 and 78; they were then retensioned and
restrained by clamps 84 and 85.
Tube 10' was centered over the stripped region as
shown in Fig. 10, and the fibers were tacked to the ends of
the tube as described above using Dymax 911 W curable
adhesive. A small amount 87 of the adhesive was carefully
applied to one side of fibers 22' and 24' at each end of
the tube to ensure the presence of openings 88 and 90. The
adhesive was exposed to a*Dymax PC-3 W light source for
thirty seconds at each end of the tube. The fiber pigtails
extending from the coupler preform were color coded. At
* Trade-mark
- 26 - l 3 3 5 ~ 7 0
this time the fibers withi~ the capillary tube were
visually checked for twists. Also, a beam of HeNe laser
light was launched into that end of fiber 22' protruding
from clamp 84. The radiation of light from the remaining
end of that fiber indicated that no partial twist was
present. An orientation mar~ was placed on the upper
~urface of tube 10'.
Coupler preform 91 was inserted through ring burner
94. With the orientation ma~k facing the operator, the
ends of the preform were secured in chucks 92 and 93.
Vacuum attachments 96 and 101 were attached to the preform
ends as shown in Fig. 13. Jaws 99 were clamped on tubing
98 to apply a vacuum to th lower portion of coupler
preform 91 while the upper end of the preform was vented.
This "air wash" was contin~d for approximately thirty
seconds. Jaws 104 were then clamped against tubing 103 to
apply to the upper portion of ?reform 91 a vacuum that was
allowed to stabilize at ap~rcx1~ately 53 cm (21 inches) of
Hg.
The ring burner was turne~ on for about 25 seconds to
increase the temperature of the midreqion of the tube to
the scftenins temperature of -he borosilicate glass. This
caused tube to collapse ont~ the fibers along a section of
the ~ube about 0.6 cm long. After the coupler preform
cooled for about 30 seconds, the flame was reignited, and
the collapsed region was reheated for about 16 seco~ds.
Stages 92 and 93 moved in opposite directions to increase
the coupler length by about 1.1 cm. All of the process
steps performed in the tube collapse step and the stretch
step were performed under the control of a PDP 11-73
micro-computer.
After the coupler haPd cc-led, the vacuum lines were
removed from the coupler, and a drop of Dymax 304 adhesive
was applied to each end of the capillary tube and was
exposed to W light for 30 seconds. The coupler was then
removed from the draw.
- 27 - l 3.35870
This process typically produced 3 d8 couplers that
operated at a predetermined wavelength such as 1300 nm.
Median excess device loss was about 0.3 dB, and the lowest
measured loss was 0.01 dB.
