U.S. patent number RE44,262 [Application Number 12/153,189] was granted by the patent office on 2013-06-04 for optical coupler comprising multimode fibers and method of making the same.
This patent grant is currently assigned to ITF Laboratories Inc.. The grantee listed for this patent is Nawfel Azami, Mathieu Faucher, Marc Garneau, Francois Gonthier, Lilian Martineau, Francois Seguin, Alain Villeneuve. Invention is credited to Nawfel Azami, Mathieu Faucher, Marc Garneau, Francois Gonthier, Lilian Martineau, Francois Seguin, Alain Villeneuve.
United States Patent |
RE44,262 |
Gonthier , et al. |
June 4, 2013 |
Optical coupler comprising multimode fibers and method of making
the same
Abstract
An optical coupler is provided. It has a bundle of multimode
fibers with a few-mode fiber in its centre. Such bundle is fused at
one end which is the output end for the signal that is transmitted
by the few-mode fiber. To make the coupler, this output end of the
bundle is aligned and spliced with a large area core double clad
fiber while preserving the modal content of the feed-through. A
method for making such optical coupler is also provided. It
includes the steps of bundling a central few-mode fiber with a
plurality of multimode fibers and then fusing one end of such
bundle and aligning it and splicing with a large core double clad
fiber, while preserving fundamental mode transmission from one to
the other.
Inventors: |
Gonthier; Francois (Quebec,
CA), Martineau; Lilian (Grenoble, CA),
Seguin; Francois (Baie-d'urfe, CA), Villeneuve;
Alain (Mont-Royal, CA), Faucher; Mathieu (Quebec,
CA), Azami; Nawfel (Rabat, MA), Garneau;
Marc (St-Laurent, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Gonthier; Francois
Martineau; Lilian
Seguin; Francois
Villeneuve; Alain
Faucher; Mathieu
Azami; Nawfel
Garneau; Marc |
Quebec
Grenoble
Baie-d'urfe
Mont-Royal
Quebec
Rabat
St-Laurent |
N/A
N/A
N/A
N/A
N/A
N/A
N/A |
CA
CA
CA
CA
CA
MA
CA |
|
|
Assignee: |
ITF Laboratories Inc.
(Montreal, CA)
|
Family
ID: |
34549937 |
Appl.
No.: |
12/153,189 |
Filed: |
May 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
10694717 |
Oct 29, 2003 |
7046875 |
May 16, 2006 |
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Current U.S.
Class: |
385/28; 385/45;
385/43; 385/27; 385/50 |
Current CPC
Class: |
G02B
6/02009 (20130101); G02B 6/2821 (20130101); G02B
6/03622 (20130101); G02B 6/14 (20130101) |
Current International
Class: |
G02B
6/26 (20060101); G02B 6/42 (20060101) |
Field of
Search: |
;385/28,43,27,45,50
;359/341.1,341.3,341.33,345 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
The Photonics Design and Applications Handbook 2002 Fiber Optics:
Understanding the Basics (Lucent Technologies). cited by applicant
.
Applications Note No. M105 (Oxford Electronics Ltd.) Jun. 1, 1999.
cited by applicant.
|
Primary Examiner: Lepisto; Ryan
Attorney, Agent or Firm: Brouillette & Partners Cartier;
Francois Brouillette; Robert
Claims
The invention claimed is:
1. An optical coupler which comprises: (a) a bundle of a plurality
of multimode fibers having a few-mode fiber in the center, said
few-mode fiber being a signal fiber through which an optical signal
is transmitted; (b) a large area core double clad fiber (LACDCF)
having an inner cladding and an outer cladding with a lower
refractive index, and having an end portion terminating with an
input end the inner cladding of which has a predetermined
circumference, into said input end of the LACDCF the optical signal
is to be transmitted from said few-mode fiber; (c) said bundle
having a fused end portion with an output end having a periphery
that fits within the circumference of the inner cladding of the
input end of the LACDCF; and (d) said output end of the bundle
being aligned and spliced with the input end of the LACDCF in such
a way as to preserve fundamental mode transmission from the
few-mode fiber to the LACDCF.
2. An optical coupler according to claim 1, in which the outer
cladding of the LACDCF is a polymer cladding and said polymer outer
cladding is removed from the end portion of the LACDCF.
3. An optical coupler according to claim 1, in which the multimode
fibers are tapered before being fused at the output end, in order
to fit within the circumference of the inner cladding of the input
end of the LACDCF.
4. An optical coupler according to claim 3, in which the multimode
fibers are tapered and fused in such a way as not to affect the
fundamental mode transmission in the core of the few-mode
fiber.
5. An optical coupler according to claim 3, in which the tapering
of the multimode fibers reduces the core of the few-mode fiber to
the fundamental mode size near the output end of the bundle, when
said core bad been expanded prior to bundling.
6. An optical coupler according to claim 1, in which the few-mode
fiber at the output end is a fiber with a core expanded from a
single mode fiber.
7. An optical coupler according to claim 1, in which the plurality
of multimode fibers is placed essentially symmetrically around the
few-mode fiber within the bundle.
8. An optical coupler according to claim 1, in which at least one
of the plurality of multimode fibers in the bundle is replaced by a
dummy fiber.
9. A method of forming an optical coupler, which comprises; (a)
bundling a central few-mode fiber with a plurality of surrounding
multimode fibers so that the surrounding multimode fibers are
positioned essentially symmetrically around the central few-mode
fiber, thereby forming a bundle of said fibers having an output
end; (b) providing a large area core double clad fiber (LACDCF)
having an inner cladding and an outer cladding with a lower
refractive index, and having an end portion terminating with an
input end where the inner cladding of the LACDCF has a given
circumference; (c) fusing the output end of the bundle so that its
periphery fits within the circumference of the inner cladding of
the input end of the LACDCF; and (d) splicing the fused output end
of the bundle to the input end of the LACDCF in such a manner that
the core of the few-mode fiber is precisely modally aligned with
the core of the LACDCF so as to preserve fundamental mode
transmission from the few-mode fiber to the LACDCF.
10. Method according to claim 9, in which the outer cladding of the
LACDCF is a polymer cladding and the method includes removing said
polymer outer cladding from the end portion of the LACDCF prior to
splicing.
11. Method according to claim 9, in which the multimode fibers at
the output end of the bundle are tapered prior to being fused.
12. Method according to claim 11, in which the tapering is done
according to the following maximum taper ratio:
R=.rho..sub.o/.rho..sub.i=NA.sub.MM/NA.sub.DCF where R is the
maximum taper radio .rho..sub.o is the final diameter of the
multimode fiber .rho..sub.i is the initial diameter of the
multimode fiber NA.sub.MM is the numerical aperture of the
multimode fiber NA.sub.DCF is the numerical aperture of the LACDCF
inner cladding waveguide.
13. Method according to claim 11, in which the tapering of the
multimode fibers followed by fusing of the bundle in done so as not
to affect the modal size of the core of the few-mode fiber.
14. Method according to claim 11, in which the tapering of the
multimode fibers followed by fusing of the bundle decreases the
modal size of the core of the few-mode fiber, when said core had
been expanded prior to bundling.
15. Method according to claim 9, in which the few-mode fiber has a
core near the output end of the bundle, which is expanded from an
initial single-mode fiber core.
16. Method according to claim 15, in which the expansion is done by
means of a mode converter to increase the size of the core.
17. Method according to claim 15, in which the expansion is done by
beating to a high temperature such that germanium present in the
core diffuses into the cladding, thereby increasing the size of the
core and of the mode.
18. Method according to claim 9, in which the cores of the few-mode
fiber and of the LACDCF are precisely modally aligned by launching
the fundamental mode of the few-mode fiber while monitoring the
modal content at the input end of the LACDCF by means of a near
field measurement device and aligning the output end of the bundle
and the input end of the LACDCF unit a Gaussian mode field is
obtained.
19. Method according to claim 18, in which the LACDCF is kept
straight or under a small tension, when monitoring the modes by the
near field measurement device, and the measurement is done at or
near operational wavelength.
20. Method according to claim 19, in which the coupler is packaged
by bonding it to a suitable substrate to preserve the alignment of
the components.
.Iadd.21. An optical fiber coupler device comprising: a) a
double-clad fiber comprising a large area central core, an inner
cladding having a first refractive index, and an outer cladding
having a second refractive index lower than said first refractive
index; b) a fiber bundle comprising a plurality of multimode fibers
coupled to said inner cladding of said double-clad fiber, and a
central fiber having a few-mode central core, said central fiber
being coupled to said double-clad fiber such that said few-mode
central core is coupled to said large area central core and such as
to preserve fundamental mode transmission between said few-mode
central core of said central fiber and said large area central core
of said double-clad fiber; wherein said fiber bundle is being
tapered to a reduced cross sectional area prior to being coupled to
the double-clad fiber such that said tapered fiber bundle fits
within a circumference of said inner cladding of said double-clad
fiber..Iaddend.
.Iadd.22. An optical fiber assembly comprising: a) a fiber bundle
comprising a plurality of multimode pump fibers and a central
few-mode fiber, said few-mode fiber comprising a few-mode central
core, said bundle having an outer diameter; and b) a double-clad
fiber comprising a large area central core, an inner cladding, and
an outer cladding, said inner cladding having an outer diameter
that is at least equal to said outer diameter of said fiber bundle;
wherein said fiber bundle is spliced to said double-clad fiber such
that said few-mode central core and said large area central core
are optically coupled such as to preserve fundamental mode
transmission between said central cores, and such that optical
power in said multimode pump fibers of said fiber bundle is coupled
into said inner cladding of said double-clad fiber..Iaddend.
.Iadd.23. An optical fiber coupler device comprising: a) a
double-clad fiber comprising a large area central core, an inner
cladding, and an outer cladding; b) a plurality of multimode fibers
optically coupled to said inner cladding of said double-clad fiber;
and c) a few-mode fiber comprising a few-mode central core, said
few-mode fiber being coupled to said double-clad fiber such that
the few-mode central core is coupled to said large area central
core of said double-clad fiber and such as to preserve fundamental
mode transmission between said few-mode central core of said
few-mode fiber and said large area central core of said double-clad
fiber; wherein said plurality of multimode fibers and said few-mode
fiber are bundled together into a fiber bundle, wherein said
few-mode fiber is located at the center of said fiber bundle, and
wherein said fiber bundle is being tapered to a reduced cross
sectional area prior to being coupled to said double-clad fiber
such that said tapered fiber bundle fits within a circumference of
said inner cladding of said double-clad fiber..Iaddend.
.Iadd.24. An optical fiber assembly comprising: a) a fiber bundle
comprising a plurality of multimode pump fibers, and a central
fiber comprising a few-mode central core, said fiber bundle having
an outer diameter; and b) a double-clad fiber comprising a large
area central core, an inner cladding, and an outer cladding, said
inner cladding having an outer diameter that is at least equal to
said outer diameter of said fiber bundle, wherein said fiber bundle
is spliced to said double-clad fiber such that said central cores
are optically coupled such as to preserve fundamental mode
transmission between said central cores, and such that optical
power in said multimode pump fibers is coupled into said inner
cladding of said double-clad fiber..Iaddend.
.Iadd.25. The optical fiber assembly according to claim 24, wherein
said fiber bundle is tapered and spliced to said double-clad fiber
so as to not affect fundamental mode transmission from between said
central cores..Iaddend.
.Iadd.26. The optical fiber assembly according to claim 25, wherein
said few-mode central core is narrowed in a first region where said
fiber bundle is tapered, and wherein said few-mode central core is
expanded in a second region where said fiber bundle is spliced to
said double-clad fiber..Iaddend.
.Iadd.27. The optical fiber assembly according to claim 26, further
comprising core expanding means for expanding said few-mode central
core in said second region..Iaddend.
.Iadd.28. An optical fiber assembly comprising: a) a fiber bundle
comprising a plurality of multimode pump fibers, and a central
single-mode fiber comprising a single-mode central core, said fiber
bundle having an outer diameter; and b) a double-clad fiber
comprising a large area core, an inner cladding, and an outer
cladding, said inner cladding having an outer diameter that is at
least equal to said outer diameter of said fiber bundle, wherein
said fiber bundle is spliced to said double-clad fiber such that
said central cores are optically coupled such as to preserve
fundamental mode transmission between said central cores, and such
that optical power in said multimode pump fibers of said fiber
bundle is coupled into said inner cladding of said double-clad
fiber; and wherein said fiber bundle comprises a mode converter
coupled to said single-mode fiber for increasing a size of said
single-mode central core of said single-mode fiber to be compatible
with said large area central core of said double-clad
fiber..Iaddend.
.Iadd.29. The optical fiber assembly according to claim 28, wherein
said mode converter comprises a fiber having a few-mode central
core..Iaddend.
.Iadd.30. The optical fiber assembly according to claim 28, wherein
said mode converter comprises diffusion means for diffusing said
single-mode central core to thereby cause said single-mode central
core to become few-moded in a region adjacent to where said fiber
bundle is spliced to said double-clad fiber..Iaddend.
.Iadd.31. An optical fiber assembly comprising: a) a fiber bundle
comprising a plurality of multimode pump fiber, and a central
few-mode fiber comprising a few-mode central core, said fiber
bundle having an outer diameter; and b) a double-clad fiber
comprising a large area central core, an inner cladding, and an
outer cladding, said inner cladding having an outer diameter that
is at least equal to said outer diameter of said fiber bundle;
wherein said fiber bundle is spliced to said double-clad fiber such
that said few-mode central core of said few-mode fiber is optically
coupled to said large area central core of said double-clad fiber
such as to preserve fundamental mode transmission between said
central cores, and such that optical power in said multimode pump
fibers of said fiber bundle is coupled into said inner cladding of
said double-clad fiber..Iaddend.
.Iadd.32. An optical fiber assembly comprising: a) a fiber bundle
comprising a plurality of multimode pump fiber, and a central fiber
comprising a few-mode central core, said fiber bundle having an
outer diameter; and b) a double-clad fiber comprising a large area
central core, an inner cladding, and an outer cladding, said inner
cladding having an outer diameter that is at least equal to said
outer diameter of said fiber bundle; wherein said fiber bundle is
spliced to said double-clad fiber such that said few-mode central
core of said central fiber is optically coupled to said large area
central core of said double-clad fiber such as to preserve
fundamental mode transmission between said central cores, and such
that optical power in said multimode pump fibers of said fiber
bundle is coupled into said inner cladding..Iaddend.
.Iadd.33. An optical coupler according to claim 1, in which the
bundle is tapered before being fused at the output end in order to
fit within the circumference of the inner cladding of the input end
of the LACDCF..Iaddend.
.Iadd.34. An optical coupler according to claim 33, in which the
bundle is tapered and fused in such a way as not to affect the
fundamental mode transmission in the core of the few-mode
fiber..Iaddend.
.Iadd.35. An optical coupler according to claim 33, in which the
tapering of the bundle reduces the core of the few-mode fiber to
the fundamental mode size near the output end of the bundle, when
said core had been expanded prior to bundling..Iaddend.
Description
FIELD OF THE INVENTION
This invention relates generally to optical fiber couplers. More
particularly, the invention pertains to optical fiber couplers for
coupling a bundle of multimode fibers, containing a few-mode fiber
in their centre, to a large core area double clad fiber. The
invention also provides a method for making such coupler.
BACKGROUND OF THE INVENTION
Multimode optical fibers are used in a large number of
applications, such as communications networks, sensors systems,
avionic and aerospace industry, medical instruments, fiber bundles,
and fiber amplifiers and lasers. One of the basic components in
most of these applications is the multimode fiber coupler, that can
take several different forms, such as the power splitter, the tap
coupler, the star coupler or the power combiner. All these
components essentially take several multimode fibers and bundle
them together by either mechanically holding them or twisting them
together, and the structure is fused and/or tapered in order to
induce coupling between the fibers from the input to the output.
The basic description of this coupling is given in U.S. Pat. No.
4,291,940 of Kawasaki et al. It discloses that if two multimode
fibers are placed side by side and then fused together using a heat
source, there is some optical power transfer from one fiber to the
other. Such transfer can be increased as the structure is pulled
and tapered.
This basic fused-tapered concept was used in several subsequent
patents such as U.S. Pat. Nos. 4,392,712 and 4,330,170 where it
became apparent that this procedure could also be used for more
than two fibers, thus creating M.times.N fused taper bundles, where
M is the input number of fibers and N is the output number of
fibers. Moreover, the fuse and taper process received some further
improvements such as described in U.S. Pat. Nos. 4,426,215 and
4,550,974 where several techniques are disclosed to improve the
uniformity of the power distribution in the fused-tapered multimode
fiber bundles. In particular, U.S. Pat. No. 4,550,974 describes a
process presently known in the art as the "cut and fuse" process
where a fused tapered multimode fiber bundle is cut and then fused
together again to produce a better mode scrambling effect and thus
better uniformity. From this process, it quickly became apparent
that one did not need to fuse the same two coupler halves together,
but one could put together two different coupler halves, thereby
creating another way of making M.times.N couplers.
As applications of multimode fiber evolved, there came another
application that can benefit from this process. The double clad
fiber amplifiers or lasers use a type of fiber, the double clad
fiber (DCF), that has a single-mode core doped with rare-earth
ions, such as ytterbium, erbium or neodinium, that is surrounded by
an optical cladding of far larger diameter. This cladding is a
highly multimode waveguide and it is surrounded by another optical
cladding having a lower refractive index, which may be a polymer
cladding. To amplify an optical signal propagating through the DCF
core, one needs to optically pump the rare-earth ions. This pump
optical power can be injected in the core in the same manner as in
single-mode fiber amplifiers, but the purpose of the double clad is
that the pump power can be injected into the inner cladding which
surrounds the core. Because some of the cladding modes travel
through the core, they provide energy to the rare-earth ions and
enable the amplification of the signal to occur. Moreover, because
the inner cladding is far larger than the core, it is possible to
input a greater number of pump laser light and spatially multiplex
the same in the cladding, rather than wavelength or polarization
multiplex the pump laser in the core. Thus, a much greater amount
of pump power is available in DCF for the amplification than in
single-mode fiber amplifiers.
In some DCF amplifiers or lasers, the coupling is achieved by bulk
optics, coupling the pump power through lens and mirrors into the
double cladding. U.S. Pat. No. 5,864,644 describes how this can be
done with a multimode taper bundle using a similar approach as the
"cut and fuse" technique, where the second coupler half is replaced
by a DCF. The patent also describes how it is possible to include
in the bundle s single-mode fiber, that will connect to the
single-mode core of the DCF, thus allowing a signal to go through
the coupler and be amplified or reversely, if the coupler is used
in a counter pump configuration (i.e. the pump power and the signal
go in the opposite direction), to let the signal out of the
amplifier with minimum loss. A modification to this structure is
disclosed in U.S. Pat. No. 6,434,302, where it is stated that for
better performance, the tapered bundle and DCF structure must be
tapered further than the diameter of the DCF to improve mode
distribution for improved gain efficiency.
In high power amplifiers and lasers, as the power available for
pump is greater, the power output of the amplifier or laser is also
larger, to the point where the light intensity in the doped glass
becomes large enough to damage the glass or to produce undesirable
non-linear effects, such as Raman or Brilloin scattering. Thus, a
new generation of DCF fibers has been developed to address these
high power situations. These fibers have a large core area so that,
even if the power is high, the intensity in the core remains
reasonable. Even if one decreases the index step of the core
waveguide, the large core is not necessarily single-mode at the
laser wavelength. The fiber core is few-moded. One must carefully
excite the core fundamental mode to have the amplification in that
mode, that will produce the best output beam. This problem is not
addressed in U.S. Pat. Nos. 5,864,644 and 6,434,302 which deal only
with a single-mode connection. A single-mode connection is simple
because one cannot excite anything other than the fundamental mode
in the connection, even if the splice between the tapered fiber
bundle and the DCF is bad. In the few-mode case, this connection is
crucial to the proper functioning of the amplifier.
Thus, there is a need for a coupler that provides a connection of a
bundle of multimode pump fibers that have a few-mode signal fiber
in their centre, to a large area core double clad fiber
(LACDCF).
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical
coupler with a fused fiber bundle of multimode fibers having a
few-mode fiber in their centre, to be connected to LACDCF.
Another object of the present invention is to provide an input end
fused fiber periphery or fiber bundle transverse geometry that
would preserve the modal content of the few-mode core in such
coupler.
A still further object of the present invention is to provide a
method of making a coupler with the above mentioned properties,
including alignment and splicing of the fiber bundle to the
LACDCF.
Other objects and advantages of the invention will become apparent
from the following description thereof.
As is known, a single-mode fiber normally has a mode field diameter
of up to 9 .mu.m, whereas a few-mode fiber usually has a mode field
diameter of 30 50 .mu.m, while multimode fibers generally have a
core mode field diameter above 50 .mu.m. Also, in the LACDCFs the
core has a mode field diameter which is similar to that of the core
of the few-mode fiber.
In a single-mode connection, one deals with two cores that have the
same mode field diameter, because at one point in the tapering
process, the mode field diameter increases rather than decreasing.
One can thus, by tapering a bundle of fibers that include a
single-mode fiber, match the mode field of the single-mode core of
the tapered bundle to the mode of the DCF single-mode core, as
disclosed, for instance, in U.S. Pat. No. 6,434,302. This, however,
is not possible to achieve with a few-mode fiber, unless one tapers
the bundle to the point where the two fiber core becomes
single-mode. Thus, the basic difference in connecting the LACDCF to
a bundle with a few-mode fiber is that the signal transmitting the
few-mode fiber cannot simply be tapered to achieve the connection
(as in the case of single-mode fibers), and must be made to match
the modal content of the LACDCF.
Thus, in essence, according to the present invention, there is
provided an optical coupler which comprises: (a) a bundle of a
plurality of multimode fibers having a few-mode fiber in the
centre, said few-mode fiber being a signal fiber through which an
optical signal is transmitted; (b) a large area core double clad
fiber (LACDCF) having an inner cladding and an outer cladding with
a lower refractive index, which outer cladding may be made of a
polymer, said LACDCF having an end portion from which the outer
cladding is removed if it is made of a polymer, said end portion
terminating with an input end the inner cladding of which has a
predetermined circumference, into which input end of the LACDCF the
optical signal is to be transmitted; (c) said bundle having a fused
end portion with an output end having a periphery that fits within
the circumference of the inner cladding of the input end of the
LACDCF; and (d) said output end of the bundle being aligned and
spliced with the input end of the LACDCF in such a way as to
preserve fundamental mode transmission from the few-mode fiber to
the LACDCF.
The multimode fibers of the bundle end portion may also be tapered
before being fused in order to fit within the circumference of the
inner cladding of the input end of the LACDCF.
The method of the present invention essentially comprises: (a)
bundling a central few-mode fiber with a plurality of surrounding
multimode fibers so that the surrounding multimode fibers are
placed generally symmetrically around the central few-mode fiber,
thereby forming a bundle of said fibers having an output end; (b)
providing a large area core double clad fiber (LACDCF) and if it
has an outer polymer cladding, removing said outer polymer cladding
from an end portion thereof, said end portion terminating with an
input end of the LACDCF having an inner cladding of a given
circumference; (c) fusing the output end of the bundle so that its
periphery fits within the circumference of the inner cladding of
the input end of the LACDCF; and (d) splicing the fused output end
of the bundle to the input end of the LACDCF in such a manner that
the core of the few-mode fiber is precisely modally aligned with
the core of the LACDCF so as to preserve fundamental mode
transmission from the few-mode fiber to the LACDCF.
The multimode fibers at the output end of the bundle may also be
tapered prior to being fused, so as to fit within the circumference
of the inner cladding of the input end of the LACDCF.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the appended
drawings, in which:
FIG. 1 is a schematic side view representation of a coupler
arrangement in accordance with the present invention in which a
bundle of three fibers comprising two multimode fibers and a
few-mode fiber in the centre is connected to the LACDCF;
FIG. 2 is a section view of the splicing region of FIG. 1 along
line A--A;
FIG. 3 is a perspective view of the coupler shown in FIG. 1;
FIG. 4 is a schematic side view representation of a coupler in
accordance with the present invention in which a bundle of more
than two multimode fibers with a few-mode fiber in the centre is
connected to the LACDCF;
FIG. 5 is a schematic side view representation of an embodiment of
the present invention where the core of a single-mode fiber has
been expanded and connected to a core of a few-mode fiber before
splicing with the LACDCF;
FIG. 6 is a schematic side view representation of another
embodiment in which the core of a single-mode fiber has been
expanded to a few-mode fiber level just before splicing with the
LACDCF;
FIG. 7 is a schematic side view representation of an embodiment
where the core of the few-mode fiber has been expanded prior to
bundling and then fused and tapered within the bundle fusion region
to the appropriate mode size before splicing with the LACDCF;
FIGS. 8A to 8L represent a schematic view of different fiber bundle
configurations that may be used within the scope of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the invention will now be described with
reference to the appended drawings in which the same parts are
designated by the same reference numbers.
In the embodiment shown in FIG. 1, a few-mode fiber 10 is provided,
having a core 12 of 50 .mu.m diameter and a cladding 14 of 125
.mu.m diameter. This few-mode fiber 10 is bundled with two
multimode fibers 16, 18 and the bundle is fused in the fusion
region 20. The multi-mode fibers 16, 18 each have a core 22 of 105
.mu.m diameter and a cladding 24 of 125 .mu.m diameter. The three
fibers total 375 .mu.m in their longitudinal periphery before
fusion and 350 .mu.m after fusion. The fused end of this structure
was then cleaved, aligned and spliced to the end 25 of the LACDCF
26 having a large area core 28 of 50 .mu.m diameter and an inner
cladding 30 of 350 .mu.m diameter. The second outer polymer
cladding 32 was stripped from the end portion 27 of the LACDCF
prior to splicing in the splicing region 34. The polymer cladding
is stripped so that it would not burn during splicing. However, if
a non-polymeric outer cladding is used, it does not need to be
stripped from the inner cladding near the splicing region. Once the
coupler is thus made, it is normally packaged by bonding it to a
suitable substrate to preserve the alignment of the components.
FIG. 2 illustrates a transverse section view of the coupler
arrangement of FIG. 1 along line A--A, namely essentially at the
splicing position. The few-mode fiber 10 having its core 12 bundled
and fused with multimode fibers 16 and 18, is aligned and spliced
with the LACDCF 26 having a large area core 28 (shown in FIG. 1)
that corresponds to core 12 in its modal content. When splicing the
fused bundle of fibers 10, 16 and 18 with the LACDCF fiber 26, the
periphery of the bundle is adapted to fit within the circumference
of the inner cladding 30 of the LACDCF and, if necessary, should be
tapered to achieve such size. This periphery does not need to cover
the entire surface of end 25 of the inner cladding 30 of the
LACDCF, but what is important is that the bundle and the few-mode
fiber be so aligned with the LACDCF as to preserve the fundamental
mode transmission from the few-mode fiber to the LACDCF.
Essentially, this means that the core 12 should be precisely
modally aligned with the large area core 28. This can be done by
launching the fundamental mode of the few-mode fiber and monitoring
the modal content at the input of the LACDCF with a near-field
measurement device, such as a camera that images the fiber endface
through an appropriate lens. One then aligns the bundle and the
LACDCF until a Gaussian mode field is obtained. The splice is then
made and the modal field is checked again to verify that the modal
content does not change. If the modal content has changed or was
lost because of the splice, this may be due to stresses in the
splice. The splice then needs to be reheated and reworked to
optimize the modal content. When monitoring the modes, the LACDCF
fiber should be straight or under a small amount of tension to
prevent mode coupling that would effect the measurement. Such
measurement should also be done at the wavelength of the operation
or at a wavelength very close to it in order to produce best
results.
FIG. 3, which represents a perspective view of the coupler of the
present invention, shows that the few-mode fiber 10 is positioned
in the middle between multi-mode fibers 16 and 18 and is fused and
spliced at the splicing end 25 with the LACDCF fiber 26. Near the
splicing end 25, namely at the end portion 27, the polymer outer
cladding 32 has been removed, so that the splicing is done within
the circumference of the inner cladding of the LACDCF 26. The
splicing would still be done within the circumference of the inner
cladding of the LACDCF 26, even if a non-polymeric outer cladding
were used, however, such outer cladding need not be removed from
the end portion 27.
It is also possible to use any suitable number of multi-mode fibers
16, 18 . . . N, bundled around the few-mode fiber 10. Thus, for
example, one can place six multimode fibers having a diameter of
125 .mu.m around a few mode fiber 10 also having a diameter of 125
.mu.m. These fibers would be fused within the fusion region 20 to
fit within the circumference of the inner cladding 30 of the
LACDCF, and spliced within the splicing region 34 at end 25 to the
LACDCF fiber 26 while preserving fundamental mode transmission from
the few-mode fiber 10 to the LACDCF 26. As already described
previously, this is achieved with proper modal alignment of the
cores 12 and 28.
In another embodiment, illustrated in FIG. 4, one can use, for
example, seven multimode fibers 17N having a diameter of 220 .mu.m,
bundled around a 125 .mu.m few-mode fiber 10. The fibers 17N are
tapered to 125 .mu.m diameter before fusing them in the fusion
region 20. They are then spliced to the LACDCF fiber 26 in the
splicing region 34 at end 25 of the LACDCF, after alignment to
preserve the modal content of the feed-trough.
In general, when tapering the outer multimode fibers, one should
not taper them more than the ratio:
R=.rho..sub.o/.rho..sub.i=NA.sub.MM/NA.sub.DCF where R is the
maximum taper radio
.rho..sub.o is the final diameter of the multimode fiber
.rho..sub.i is the initial diameter of the multimode fiber
NA.sub.MM is the numerical aperture of the multimode fiber
NA.sub.DCF is the numerical aperture of the LACDCF inner cladding
waveguide.
When tapering the outer multimode fibers, one can have any suitable
number of such fibers bundled and then fused around a few-mode
fiber, provided the above taper ratio is maintained. Some such
bundle configurations are shown in FIGS. 8A to 8L discussed
below.
Further embodiments of the invention as illustrated in FIGS. 5, 6
and 7 relate to the adjustment of the mode field diameter of the
signal fiber to the large area core of the double clad fiber.
Thus, in FIG. 5 the invention provides an adjustment of the mode
field diameter of the signal fiber 11, which is a single-mode fiber
having a 6 .mu.m core 13, to the LACDCF fiber 26 having a 50 .mu.m
core 28. This is done by providing a mode converter to increase the
size of the core as shown at 15 and splice it to a length of a few
mode fiber at 21 so as to connect it with the 50 .mu.m core 12 of
the few-mode fiber provided within the fusion region 20. Then, the
few-mode fiber is fused in the fusion region 20 with the multimode
fibers 16, 18 and spliced with the LACDCF 26 in the splicing region
34 as already described previously, so that cores 12 and 28 are
coupled to preserve the fundamental mode transmission from the few
mode fiber to the LACDCF.
In another embodiment illustrated in FIG. 6, rather than using a
mode converter to expand and splice a single-mode fiber to a length
of a few-mode fiber, one can provide a mode converter to diffuse
the core 13 of a single mode fiber 11 at 19 so that the core 13 is
diffused within the fusion region 20 near the splicing surface 25,
whereby the fiber 11 becomes few-moded over a few centimeters
leading to the splicing region 34, where the fiber is aligned and
spliced with the LACDCF 26. The core 13 is diffused at 19 to become
a 50 .mu.m core that can be aligned and spliced with core 28 of the
LACDCF 26 as described previously. This configuration is similar to
that shown in FIG. 5, except there is no transitional few-mode
fiber provided in the bundle.
The embodiment illustrated in FIG. 7 provides for an expansion of
the core 12 of the few-mode fiber 10 that has been narrowed while
tapering the fibers 17N around it, as disclosed with reference to
FIG. 4. This may happen in some cases due to a particular design of
the bundle where one must taper the few-mode fiber because the
diameter of the fused bundle is larger than the diameter of the
internal cladding of the LACDCF, and one could not taper the outer
multi-mode fibers more because of the relation of the numerical
apertures as expressed in the ratio R above. In such as case, one
can expand the core 12 of the few-mode fiber 10 using the mode
converter referred to above, for example at location 23 within the
fusion region 20, and leading to the splicing region 34. This
expansion is done so that the core of the few-mode fiber at the
splicing surface 25 will match core 28 of the LACDCF 26 and produce
the required modal content of the feed-through. It is important to
note that when additional tapering is done, the total tapering of
the pump fibers should not exceed the R ratio referred to above.
Any tapering in excess of that ratio will cause extra loss.
The mode converter mentioned above with reference to FIGS. 5, 6 and
7 is usually produced by heating the fiber to a high temperature
such that the germanium, which is present as a dopant in the fiber
core, diffuses into the cladding, thereby increasing the size of
the core and thus of the mode. At one point of such diffusion
process, the mode field diameter of the expanded core becomes equal
to that of the large area core of the DCF and at this point the
heating is stopped and the mode conversion is completed.
In the various bundles and particularly in the tapered bundles, it
is important to respect the symmetry around the few-mode fiber as
much as possible, so that the fusion process does not
asymmetrically deform the core of the few-mode fiber, which would
make it difficult to produce a good splice. However, if the number
of fibers is large, for example greater than 19, one does not need
to be too careful in preserving the symmetry of the structure so
long as the signal fiber remains essentially in the center of the
fiber bundle. The deformation of the core will be negligible, even
with high fusion and tapering of the outer multimode fibers.
FIGS. 8A to 8L illustrate different fiber configurations that can
be used in accordance with the present invention within bundles to
be coupled with LACDCF. It should be noted that these
configurations are not limitative.
Thus, FIG. 8A shows a configuration of 3.times.1 or (2+1).times.1,
such as already described with reference to FIGS. 1, 2 and 3,
namely where three fibers are coupled with one LACDCF. In this
case, the middle signal fiber FMF is a few-mode fiber and the two
outer pump fibers MMF are multimode fibers.
FIG. 8B illustrates a 4.times.1 or (3+1).times.1 configuration,
again with the FMF in the middle and three MMF surrounding it in
symmetrical manner.
FIGS. 8C and 8D illustrate two symmetrical configurations of
5.times.1 or (4+1).times.1 bundles that can be used in accordance
with this invention. The FMF is located in the middle and is
symmetrically surrounded by MMF fibers.
FIGS. 8E and 8F show two 6.times.1 or (5+1).times.1 configurations.
In FIG. 8E the FMF is surrounded by 5 MMF fibers of same diameter
and in FIG. 8F the central FMF is surrounded by 5 larger MMF pump
fibers.
FIG. 8G shows a 7.times.1 or (6+1).times.1 configuration where the
central FMF is surrounded by 6 MMF pump fibers of same size.
FIG. 8H illustrates a (9+1).times.1 configuration where the central
FMF is significantly larger than the 9 surrounding MMF pump
fibers.
FIG. 8I shows a configuration of (18+1).times.1 where the central
FMF is larger than the 18 surrounding MMF pump fibers. In this
case, only a few of the surrounding fibers have been identified as
MMF fibers, but all of them can be MMF fibers, although if not all
ports are needed, some of them can be replaced by dummy fibers,
namely pure silica coreless fibers.
FIG. 8J illustrates a 19.times.1 or (18+1).times.1 configuration
where the central FMF is surrounded by 18 MMF fibers of same size.
Again, if some of the MMF ports are not needed, they can be
replaced by dummy fibers and this applies to all
configurations.
FIG. 8K shows a 19.times.1 or (18+1).times.1 configuration where a
large diameter FMF is surrounded by 18 MMF pump fibers.
Finally, FIG. 8L illustrates a 37.times.1 or (36+1).times.1
configuration where a central FMF is surrounded by 36 MMF pump
fibers of same diameter.
By way of example, for a reduction of the pump fibers by a factor
of 2, namely reducing 125 .mu.m diameter pump fibers to 65 .mu.m,
one can place 9 fibers around the 125 .mu.m signal fiber as
illustrated in FIG. 8H. One can also add another layer of the pump
fibers to form a configuration of 18 pump fibers around one signal
fiber as shown in FIG. 8I. One can further add an additional layer
of pump fibers to produce 27 pump fibers surrounding a signal fiber
as shown in FIG. 8K. With 27 pump fibers, the configuration fits
within a 400 .mu.m diameter LACDCF.
In a further example, if the pump fibers are 220 .mu.m in diameter
with 0.22 numerical aperture, one can reduce them to 125 .mu.m
diameter and bundle them around a 125 .mu.m central signal fiber.
This can produce configurations where all fibers are of equal size
as illustrated in FIGS. 8A, 8B, 8C, 8D, 8E, 8G, 8J and 8L. The
configurations of 7.times.1 shown in FIG. 8G, 19.times.1 shown in
FIG. 8J and 37.times.1 shown in FIG. 8L are close-pack
configurations, meaning that there is essentially no space left
between the fibers.
It should be noted that the invention is not limited to the
specific embodiments described above, but that various
modifications obvious to those skilled in the art may be made
without departing from the invention and the scope of the following
claims.
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