U.S. patent application number 09/971875 was filed with the patent office on 2002-03-21 for methods and devices for reducing splice loss in an optical transmission line.
Invention is credited to Veng, Torben E..
Application Number | 20020034364 09/971875 |
Document ID | / |
Family ID | 24676522 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034364 |
Kind Code |
A1 |
Veng, Torben E. |
March 21, 2002 |
Methods and devices for reducing splice loss in an optical
transmission line
Abstract
Techniques and devices are described for reducing splice loss in
an optical transmission line. According to one technique, an
electric arc is generated from an arc current, the arc current
having a level and duration sufficient to produce an electric arc
with an intensity and duration sufficient to achieve a desired
splicing temperature at a splice point between a first optical
fiber and a second optical fiber positioned within the electric
arc. The electric arc is used to splice together the first and
second optical fibers. After the fibers have been spliced together,
the level of the arc current is ramped downward over time, thereby
creating a downward ramp in temperature at the splice point from
the splicing temperature to a cooler temperature, the downward ramp
in temperature being shaped to reduce splice loss. The techniques
and devices described herein are suitable for use with various
splice combinations.
Inventors: |
Veng, Torben E.; (Broendby,
DK) |
Correspondence
Address: |
Peter H. Priest, Esq.
Priest & Goldstein, PLLC
529 Dogwood Drive
Chapel Hill
NC
27516
US
|
Family ID: |
24676522 |
Appl. No.: |
09/971875 |
Filed: |
October 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09971875 |
Oct 4, 2001 |
|
|
|
09667031 |
Sep 21, 2000 |
|
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Current U.S.
Class: |
385/96 |
Current CPC
Class: |
G02B 6/03644 20130101;
G02B 6/03627 20130101; G02B 6/02261 20130101; G02B 6/2551
20130101 |
Class at
Publication: |
385/96 |
International
Class: |
G02B 006/255 |
Claims
I claim:
1. A method for reducing splice loss in an optical transmission
line, comprising: (a) generating an electric arc from an arc
current, the arc current having a level and duration sufficient to
produce an electric arc with an intensity and duration sufficient
to achieve a desired splicing temperature at a splice point between
a first optical fiber and a second optical fiber positioned within
the electric arc; (b) using the electric arc to splice together the
first and second optical fibers; and (c) ramping the level of the
arc current downward over time, thereby creating a downward ramp in
temperature at the splice point from the splicing temperature to a
cooler temperature, the downward ramp in temperature being shaped
to reduce splice loss.
2. The method of claim 1, wherein in step (c), the ramping of the
arc current is performed automatically by an arc current
controller.
3. The method of claim 2, wherein the arc current controller is
programmable, and wherein the method further includes the following
step performed before step (a): programming the arc current
controller to create a downward ramp in temperature at the splice
point from the splicing temperature to a cooler temperature after
the first and second fibers have been spliced together, the
downward ramp in temperature being shaped to reduce splice
loss.
4. The method of claim 1, wherein the first optical fiber is
dispersion-compensating fiber.
5. The method of claim 4, wherein the second optical fiber is a
bridge fiber.
6. The method of claim 1, wherein the first optical fiber is
inverse dispersion fiber.
7. The method of claim 6, wherein each of the first and second
optical fibers is inverse dispersion fiber.
8. A method for reducing splice loss in an optical transmission
line, comprising: (a) generating an electric arc from an arc
current, the arc current having a level and duration sufficient to
produce an electric arc with an intensity and duration sufficient
to achieve a desired first splicing temperature at a first splice
point between a first optical fiber and a bridge fiber positioned
within the electric arc; (b) using the electric arc to splice
together the first optical fiber and the bridge fiber; and (c)
ramping the level of the arc current downward over time, thereby
creating a downward ramp in temperature at the first splice point
from the first splicing temperature to a cooler temperature, the
downward ramp in temperature being shaped to reduce splice loss;
(d) removing the first optical fiber and the bridge fiber from the
electric arc; (e) adjusting the arc current to produce an electric
arc with an intensity and duration sufficient to achieve a desired
second splicing temperature at a second splice point between the
bridge fiber and a second optical fiber positioned within the
electric arc; (f) using the electric arc to splice together the
bridge fiber and the second optical fiber; and (g) ramping the
level of the arc current downward over time, thereby creating a
downward ramp in temperature at the second splice point from the
second splicing temperature to a cooler temperature, the downward
ramp in temperature being shaped to reduce splice loss.
9. The method of claim 8, wherein the first optical fiber is
dispersion-compensating fiber.
10. The method of claim 8, wherein the first optical fiber is
inverse dispersion fiber.
11. The method of claim 10, wherein each of the first and second
optical fibers is inverse dispersion fiber.
12. A splicer, comprising: a chassis; a pair of arc electrodes
mounted to the chassis for generating an electric arc; a variable
current source connected to the pair of arc electrodes, the
variable current source providing as an output a current for
driving the pair of arc electrodes and creating an electric arc of
sufficient intensity and duration to achieve a desired splicing
temperature at a splice point between first and second optical
fibers positioned within the electric arc; first and second fiber
routing guides mounted to the chassis for holding first and second
fibers to be spliced together; a controller connected to the
variable current source for automatically creating a downward ramp
of the arc current after the first fiber has been spliced to the
second fiber, thereby creating a downward ramp in temperature at
the splice point from the splicing temperature to a cooler
temperature, the downward ramp in temperature being shaped to
reduce splice loss.
13. The splicer of claim 12, wherein the controller is
programmable.
14. An optical transmission line comprising: a first optical fiber
spliced to a second optical fiber at a splice point, the first
optical fiber being spliced to the second optical fiber using an
electric arc generated from an arc current having a level and
duration sufficient to produce an electric arc with an intensity
and duration sufficient to achieve a desired splicing temperature
at the splice point, the level of the arc current being ramped
downward over time after splicing, thereby creating a downward ramp
in temperature at the splice point from the desired splicing
temperature to a cooler temperature, the downward ramp in
temperature being shaped to reduce splice loss.
15. The optical transmission line of claim 14, wherein the first
optical fiber is dispersion-compensating fiber.
16. The optical transmission line of claim 15, wherein the second
optical fiber is a bridge fiber.
17. The optical transmission line of claim 14, wherein the first
optical fiber is inverse dispersion fiber.
18. The optical transmission line of claim 17, wherein each of the
first and second optical fibers is inverse dispersion fiber.
19. An optical transmission line comprising: a first optical fiber
spliced to a first end of a bridge fiber at a first splice point
and a second optical fiber spliced to a second end of the bridge
fiber at a second splice point, the first optical fiber being
spliced to the bridge fiber using an electric arc generated from an
arc current having a level and duration sufficient to produce an
electric arc with an intensity and duration sufficient to achieve a
desired first splicing temperature at the first splice point, the
level of the arc current being ramped downward over time after the
first optical fiber is spliced to the bridge fiber, thereby
creating a downward ramp in temperature at the first splice point
from the first splicing temperature to a cooler temperature, the
downward ramp in temperature being shaped to reduce splice loss,
the second optical fiber being spliced to the bridge fiber using an
electric arc generated from an arc current having a level and
duration sufficient to produce an electric arc with an intensity
and duration sufficient to achieve a desired second splicing
temperature at the second splice point, the level of the arc
current being ramped downward over time after the second optical
fiber is spliced to the bridge fiber, thereby creating a downward
ramp in temperature at the second splice point from the second
splicing temperature to a cooler temperature, the downward ramp in
temperature being shaped to result in a reduction in splice
loss.
20. The optical transmission line of claim 19, wherein the first
optical fiber is dispersion-compensating fiber.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/667,031, filed Sep. 21, 2000, and assigned
to the assignee of the present application, the disclosure and
drawings of which are hereby incorporated by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to improvements to
optical fiber transmission lines, and more particularly to
advantageous aspects of methods and devices for reducing splice
loss in an optical transmission line.
[0004] 2. Description of the Prior Art
[0005] As optical data transmission lines increase in length and in
the amount of data that they carry, there is increasing interest in
the development of new types of optical fiber and in techniques
that can be used to refurbish already existing transmission lines.
One important parameter of an optical transmission line is the
amount of signal dispersion resulting from the optical
characteristics of the materials used to construct the line. A new
class of fibers has recently been developed known as
dispersion-compensating fiber (DCF), which has a steeply sloped,
negative dispersion characteristic.
[0006] One application for DCF fiber is to upgrade already existing
fiber optic communication links. These already existing links are
typically constructed using standard single-mode fibers (SMF)
having dispersion characteristics that are optimized for operation
at a signal wavelength of 1310 nm. However, certain applications
require optimization of a communication link for operation at a
longer wavelength, particularly where the communication link spans
great distances. For example, one wavelength-division multiplexing
(WDM) technique currently in use requires optimization of the link
for operation at a wavelength of 1550 nm.
[0007] It is possible to refurbish an already existing SMF fiber
transmission line optimized for operation at a given wavelength,
such as 1310 nm, by splicing a length of DCF fiber into the
transmission line. The length of the DCF fiber added to the SMF
fiber transmission line is precisely calculated to produce an
adjustment in the overall dispersion characteristics of the line
such that it is now optimized for operation at a different desired
wavelength, such as 1550 nm. A suitable technique for precisely
calculating a length of DCF fiber to be spliced into an already
existing line in order to optimize the line for operation at a
different wavelength is disclosed in U.S. patent application Ser.
No. 09/596,454, filed on Jun. 19, 2000, assigned to the assignee of
the present application, the drawings and disclosure of which are
hereby incorporated by reference in their entirety.
[0008] In addition to dispersion, a second important parameter for
DCF fiber is the fiber's loss value, that is, the amount of excess
signal loss resulting from the introduction of the DCF fiber into a
transmission link. Optimally, a DCF fiber should provide a highly
negative dispersion, while only introducing a small excess loss to
the fiber link. A useful index of the performance of a DCF fiber is
the so-called "figure of merit" (FOM), which is defined as the
dispersion of the fiber divided by the attenuation.
[0009] Another important issue arising in connection with DCF fiber
is the excess loss that results when DCF fiber is spliced to a
standard single-mode fiber (SMF). To obtain a highly negative
dispersion, DCF fiber uses a small core with a high refractive
index, having a mode-field diameter of approximately 5.0 .mu.m at
1550 nm, compared with the approximately 10.5 .mu.m mode-field
diameter of SMF fiber at 1550 nm. The difference in core diameters
results in significant signal loss when a fusion splicing technique
is used to connect DCF fiber to SMF fiber. It is possible to reduce
the amount of signal loss by choosing splicing parameters that
allow the core of the DCF fiber to diffuse, thereby causing the
mode-field diameter of the DCF core to taper outwards, resulting in
a funneling effect. However, the amount and duration of the heat
required to produce the funneling effect result in an undesirable
diffusion of dopant in the ring of refractive material surrounding
the DCF fiber core. This diffusion of ring dopant limits the amount
of splice loss reduction that can be obtained using a mode-field
expansion technique. For example, using DCF fiber with a FOM of 200
ps/nm/dB, the splice loss typically cannot be reduced below 0.7-0.8
dB when splicing directly to SMF fiber.
[0010] There is thus a need for improved techniques for splicing
DCF fiber to SMF fiber that reduces splice loss below current
limits.
SUMMARY OF THE INVENTION
[0011] The above-described issues and others are addressed by the
present invention, aspects of which provide methods and devices for
reducing splice loss in an optical transmission line. According to
one aspect of the invention, an electric arc is generated from an
arc current, the arc current having a level and duration sufficient
to produce an electric arc with an intensity and duration
sufficient to achieve a desired splicing temperature at a splice
point between a first optical fiber and a second optical fiber
positioned within the electric arc. The electric arc is used to
splice together the first and second optical fibers. After the
fibers have been spliced together, the level of the arc current is
ramped downward over time, thereby creating a downward ramp in
temperature at the splice point from the splicing temperature to a
cooler temperature, the downward ramp in temperature being shaped
to reduce splice loss.
[0012] Additional features and advantages of the present invention
will become apparent by reference to the following detailed
description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows a transverse cross section, not drawn to scale,
of a length of dispersion-compensating fiber (DCF).
[0014] FIG. 2 shows a refractive index profile of the DCF fiber
shown in FIG. 1.
[0015] FIG. 3 shows an axial cross section, not drawn to scale, of
a transmission line fabricated from a length of DCF fiber that has
been spliced to a length of single-mode fiber (SMF).
[0016] FIG. 4 shows a diagram, not drawn to scale, of an optical
fiber transmission line according to a first aspect of the present
invention, fabricated from a length of DCF fiber, a length of a
bridge fiber (BF), and a length of SMF fiber that have been spliced
together.
[0017] FIG. 5 shows a transverse cross section, not drawn to scale,
of a BF fiber suitable for use in the transmission line shown in
FIG. 4.
[0018] FIG. 6 shows a refractive index profile of the BF fiber
shown in FIG. 5.
[0019] FIG. 7 shows an axial cross section, not drawn to scale, of
the optical fiber transmission line shown in FIG. 4.
[0020] FIG. 8 shows a bar graph illustrating the splice loss
distribution for the splice between the DCF fiber and the BF fiber
in the transmission line illustrated in FIG. 4.
[0021] FIG. 9 shows a bar graph illustrating the splice loss
distribution for the splice between the BF fiber and the SMF fiber
in the transmission line illustrated in FIG. 4.
[0022] FIG. 10 shows a diagram of a suitable furnace arrangement
for heating the splice between the DCF fiber and the BF fiber
according to a further aspect of the present invention.
[0023] FIG. 11 shows a table setting forth the setpoints used in
heating the splice between the DCF fiber and the BF fiber.
[0024] FIG. 12 shows a graph illustrating a technique according to
a further aspect of the present invention for determining a maximum
temperature to which the splice between the DCF fiber and the BF
fiber is to be heated to obtain an optimal reduction in splice
loss.
[0025] FIG. 13 shows a table setting forth a comparison of the
splice loss at the splice between the DCF and BF fiber before and
after heating the splice in accordance with the present
invention.
[0026] FIG. 14A shows an elevation view of a length of DCF fiber
and a length of BF fiber that have been spliced together and
mounted into a frame.
[0027] FIG. 14B shows a plan view of the fibers and frame shown in
FIG. 14A.
[0028] FIG. 15 shows a diagram of a system in which a laser is used
to heat the splice between the lengths of DCF and BF fibers shown
in FIGS. 14A and 14B.
[0029] FIG. 16 shows a diagram of a fusion splicer.
[0030] FIG. 17 shows a flowchart of a method for reducing splice
loss according to an aspect of the invention.
[0031] FIG. 18 shows a flowchart of a method for reducing splice
loss according to another aspect of the invention.
DETAILED DESCRIPTION
[0032] Aspects of the present invention provide an optical
transmission line with reduced splice loss and methods for
fabricating an optical transmission line with reduced splice loss.
According to one aspect of the invention, a length of bridge fiber
(BF) is introduced between a length of DCF fiber (or other suitable
first transmission fiber) and a length of SMF fiber or other
suitable second transmission fiber, such as, for example, True-Wave
fiber or Large Effective Area fiber. As described in further detail
below, the BF fiber is fabricated such that, with the use of
suitable first and second sets of splicing parameters, the BF fiber
can be spliced to both the DCF fiber and SMF fiber with
significantly reduced loss at each splice, thereby resulting in a
combined fiber line that exhibits lower splice loss than a line
fabricated from DCF and SMF fibers without a BF fiber. Splice loss
at the splice between the dispersion-compensating fiber and the
bridge fiber is then further reduced by heating the splice to a
predetermined temperature and cooling the splice using a
predetermined temperature ramp.
[0033] FIG. 1 shows a transverse cross section, not drawn to scale,
of a length of DCF fiber 10. As shown in FIG. 1, the design of the
DCF fiber 10 is based on a small diameter core 12, typically
approximately 5 .mu.m, compared with a core diameter of
approximately 10 .mu.m for SMF fiber. The DCF fiber core 12 has
been doped with a high level of germanium oxide (GeO.sub.2) to have
a high refractive index. Surrounding the core is a ring 14, which
has been doped with a high concentration of fluorine (F) to have a
low refractive index. The core 12 and ring 14 are encased in a
layer of overcladding 16.
[0034] FIG. 2 shows a refractive index profile 20 for the DCF fiber
10 shown in FIG. 1.
[0035] As shown in FIG. 2, the refractive index profile 20 includes
a central spike 22, representing the high refractive index of the
DCF fiber core 12. On either side of the spike 22 are sharp dips
24, representing the low refractive index of the ring 14. Finally,
on either side of the sharp dips 24 are flat regions 26,
representing the refractive index of the overcladding 16. The DCF
fiber 10 whose structure and refractive index profile are
illustrated in FIGS. 1 and 2 typically exhibits a dispersion at
1550 nm of approximately -100 ps/km/nm, with a loss of
approximately 0.5 dB/km.
[0036] Theory predicts that the splicing of this particular DCF
fiber 10 to a length of typical SMF fiber will result in a splice
loss of approximately 2.2 dB. This loss results from the mismatch
of the mode-field distribution in the splice region. However, this
splice loss can be reduced by using a fusion splicing technique in
which the splicing parameters are chosen to allow a tapered
mode-field expansion of the core of the DCF fiber, producing a
"funneling" effect that reduces the mismatch between the DCF fiber
core and the SMF fiber core.
[0037] Specifically, when DCF fiber is fusion spliced, splicing
parameters may be chosen such that the amount and duration of the
heat generated by the splicing process will cause the GeO.sub.2 in
the core 12 to diffuse, altering the refractive index profile of
the fiber and the mode-field distribution in the splice region. The
amount of diffusion can be controlled by optimizing the splice
parameters. Thus, using optimized splice parameters, the DCF fiber
core 12 can be tapered outward in the splice region to better match
the SMF. This is illustrated in FIG. 3, not drawn to scale, which
shows an axial cross section of a transmission line 30 fabricated
from a length of the DCF fiber 10 shown in FIG. 1 spliced to a
length of SMF fiber 32. As shown in FIG. 3, the DCF fiber core 12
tapers outwards as it approaches the splice point 34, such that it
approximates the diameter of the core 36 of the SMF fiber 32.
[0038] Using this technique, it is possible to lower the splice
loss from the theoretically predicted value of 2.2 dB to
approximately 0.7-0.8 dB. It is believed, however, that the amount
of splice loss reduction using a direct splicing technique is
limited by the high mobility of fluorine during the splicing
process. In particular, fluorine begins to diffuse at a temperature
much lower than the highest temperatures reached during fusion
splicing. Because of the relatively high concentration of fluorine
dopant in the ring 14 surrounding the core 12 in the DCF fiber 10,
the ring 14 will diffuse at a faster rate than the core 12, which
in turn tends to increase splice loss. FIG. 3, not drawn to scale,
schematically illustrates the relatively greater dispersion of the
ring 14 relative to the core 12.
[0039] One technique that is currently used to reduce splice loss
is to introduce a bridge fiber (BF) between the DCF and SMF fibers.
FIG. 4 shows a diagram of a transmission line 40 incorporating this
technique. The transmission line includes a length of DCF fiber 50
(or other suitable first transmission fiber), a length of BF fiber
60, and a length of SMF fiber 70 (or other suitable second
transmission fiber). As mentioned above, other suitable second
transmission fibers include, for example, True-Wave fiber or Large
Effective Area fiber. A first end of the BF fiber 60 is spliced to
the DCF fiber 50 at a first splicing point 80, and a second end of
the BF fiber 60 is spliced to the SMF fiber 70 at a second splicing
point 82. As discussed below, it has been found that the
introduction of the BF fiber 60 between the DCF fiber 50 and the
SMF fiber 70 may be used to reduce the splice loss to as little as
0.4 dB, which is significantly lower than the 0.7-0.8 dB splice
loss obtainable without the use of a BF fiber 60.
[0040] FIG. 5 shows a cross section of the BF fiber 60 taken
through the plane 5-5, and FIG. 6 shows a refractive index profile
90 for the BF fiber 60. The core 62 of the BF fiber is similar to
the DCF fiber core 12 illustrated above in FIGS. 1 and 2. It is
doped with GeO.sub.2 at the same concentration as the DCF fiber
core 12, and has substantially the same diameter, approximately 5
.mu.m. However, the ring 64 surround the BF fiber core 62 differs
from the ring 14 surrounding the DCF fiber core 12. First, the BF
ring 64 has a larger diameter than the DCF ring 14. Second,
although both rings 14 and 64 are doped with fluorine (F), the
concentration of fluorine in the BF ring 64 is lower than the
concentration of fluorine in the DCF ring 14. The overcladding 66
of the BF fiber 60 is similar to the overcladding 16 of the DCF
fiber 10.
[0041] Thus, the BF refractive index profile 90 shown in FIG. 6 has
a different shape than the DCF refractive index profile 20 shown in
FIG. 2. Because the BF core 62 is similar to the DCF core 12 in
diameter and dopant concentration, the central peak 92 of the BF
refractive index profile is similar to the central peak 22 of the
DCF refractive index profile 20. However, because of the larger
diameter and lower dopant concentration of the BF ring 64, compared
with the diameter and dopant concentration of the DCF ring 14, the
dips 94 on either side of the central peak 92 in the BF refractive
index profile 90 are wider and shallower than the dips 24 on either
side of the central peak 22 in the DCF refractive index profile 20.
Because the BF overcladding 66 is similar to the DCF overcladding
16, the flat outer regions 96 of the BF refractive index profile 90
are similar to the flat outer regions 26 of the DCF refractive
index profile 20.
[0042] The diameter and dopant concentration of the BF ring 64 are
chosen such that overall splice loss can be reduced by choosing a
suitable first set of splicing parameters for the splice 80 between
the DCF and BF fibers 50 and 60 and a suitable second set of
splicing parameters for the splice 82 between the BF and SMF fibers
60 and 70. FIG. 7 shows an axial cross section, not drawn to scale,
of the transmission line 40 shown in FIG. 4. As shown in FIG. 7,
because the DCF core 52 and the BF core 62 have similar diameters,
no mode-field expansion is required. Thus, it is possible to select
splicing parameters for the first splice 80 that minimize or
eliminate diffusion of the fluorine dopant in the DCF ring 54. As
further illustrated in FIG. 7, because of the relatively low
concentration of fluorine dopant in the BF ring 64, there is less
diffusion of the BF ring 64 when the BF fiber 60 is spliced to the
SMF fiber 70. It is therefore possible to select splicing
parameters for the second splice 82 that allow for the full
mode-field expansion required to match the BF core 62 to the SMF
core 72 without the splice loss associated with excessive diffusion
of the fluorine dopant.
[0043] It should be noted that the particular BF fiber design
described herein is one of a number of different designs for BF
fiber that have proven to be suitable for use with the present
invention. For example, it is possible to use a BF fiber having a
core similar to the core 62 of the above-described BF fiber, but
without the fluorine-doped ring portion 64. It will be appreciated
that alternative BF fiber designs may be used without departing
from the spirit of the present invention. Also, as mentioned above,
the present invention can be used with other splice combinations,
including, for example, combinations using True-Wave fiber or Large
Effective Area fiber in place of the SMF fiber 70.
[0044] By optimizing the splicing parameters, a first end of the BF
fiber 60 can be spliced to DCF fiber 50 with an average loss as low
as 0.17 dB, and a second end of the BF fiber 60 can be spliced to
an SMF fiber 70 with an average loss as low as 0.23 dB. Thus, using
the BF fiber 70 as a bridge fiber between the DCF fiber 50 and the
SMF fiber 60, it is possible to reduce the total splicing loss to
approximately 0.4 dB. FIG. 8 is a graph 100 showing the splice loss
distribution for the splice between the DCF fiber 50 and the BF
fiber 60. The average splice loss is 0.169 dB and the standard
deviation is 0.031 dB. FIG. 9 is a graph 102 showing the splice
loss distribution for the splice between the BF fiber 60 and the
SMF fiber 70. The average splice loss is 0.228 dB and the standard
deviation is 0.016 dB.
[0045] As mentioned above, the parameters used to splice the DCF
fiber 50 to the BF fiber 60 are different from the parameters used
to splice the BF fiber 60 to the SMF fiber 70. Fluorine dopant
begins to diffuse at a lower splice temperature than GeO.sub.2, and
also diffuses more rapidly than GeO.sub.2. Thus, the splicing of
the DCF fiber 50 to the BF fiber 60 must be done using a low fusion
current and a short fusion time. These parameters allow this first
splice 80 to be accomplished with minimal, if any, mode-field
expansion and minimal, if any, fluorine diffusion, thus minimizing
splice loss. It is also important that the electrodes of the fusion
splicer be as clean as possible, as small variations in the splice
conditions due to dirty electrodes can result in a significant
increase in the splice loss.
[0046] The splicing of the BF fiber 60 to the SMF fiber 70 is not
as critical with respect to the cleanliness of the electrodes. For
this second splice 82, a higher fusion current and a longer fusion
time are used to allow the GeO.sub.2 in the BF core 62 to diffuse,
proximate to the splice 82, such that good tapering is provided
from the BF core 62 to the SMF core 72. As shown in FIG. 9, the
splice loss has a narrow distribution with a standard deviation of
only 0.016 dB.
[0047] In accordance with a further aspect of the invention, it is
possible to reduce the splice loss between the DCF fiber 50 and the
BF fiber 60 even further. In one approach, two fibers are spliced
together and removed from the splicer. As described below, the
splice is then heated to a predetermined maximum temperature for a
predetermined period of time, and then cooled back down to room
temperature following a predetermined temperature ramp. In another
approach, also described below, the controlled ramping down of the
splice temperature takes place as part of the splicing
operation.
[0048] FIG. 10 shows a diagram of a tube furnace 110 that may be
suitably employed where a splice between two fibers is heated and
cooled in an operation separate from the initial creation of the
splice. The furnace 110 includes a ceramic tube 112 surrounding the
splice point 80 between the DCF fiber 50 and the BF fiber 60. A
heating wire 114 is coiled around the tube 112, and a power supply
116 causes a heating current to flow through the heating wire 114.
A suitable material for the heating tube 112 is Degussit ceramic,
manufactured by Friatec AG (Germany). Suitable dimensions for the
heating tube 112 include an inner circumference of 2 mm, an outer
circumference of 3 mm, and a length of 10 mm. A suitable material
for the heating wire 114 is 90/10 Pt/Rh, and a suitable wire
diameter is 0.5 mm. The length of bare fiber is approximately 2
cm.
[0049] After the DCF fiber 50 has been spliced to the BF fiber 60,
the two fibers are mounted into the furnace 110 with the splice
point 80 located inside the heating tube 112. The splice point 80
is heated to approximately 1100.degree. C. and kept at this
temperature for approximately 30 seconds. Heating of the splice 80
causes a measurable decrease in splice loss. The amount of the
decrement depends upon the particular design of DCF and BF fibers
50 and 60. It is possible for this splice loss to be maintained at
room temperature by ramping down the temperature over a time period
of approximately 90 seconds. Because of the small diameter of the
DCF and BF fibers 50 and 60 and the heating tube 112, the
relatively low specific heat of the materials used to fabricate the
fibers 50 and 60 and the heating tube 112, and the relatively large
surface area of the fibers 50 and 60 and the heating tube 112, it
is possible to implement this heating ramp simply by decreasing the
amount of current flowing through the heating wire 114, without the
need for an outside cooling mechanism. However, if needed, it would
be within the spirit of the invention to add a cooling mechanism,
such as a fan, to facilitate the cooling process. Best results are
obtained if splice loss is monitored during the heat treatment. The
loss reduction effect has been observed in all DCF designs
manufactured at Lucent Technologies, including Standard DCF,
WideBand DCF, Inverse Dispersion Fiber, and High-Slope DCF.
[0050] Returning to FIG. 4, one suitable length that has been used
for the BF fiber 70 is approximately 3 meters. The first splice 90,
between the DCF fiber 50 and the BF fiber 70 is made on a fusion
splicer using a splice time, or fusion time, of approximately 0.2
sec. A splice time significantly longer than 0.2 sec may induce
fluorine diffusion in the DCF, which in turn will increase splice
loss. Using an Ericsson splicer, the following splicing parameters
has been used to produce satisfactory results:
[0051] Pre-fusion Time=0.2 sec
[0052] Pre-fusion Current=10.0 mA
[0053] Gap=50.0 microns
[0054] Overlap=5.0 microns
[0055] Fusion Time 1=0.3 sec
[0056] Fusion Current 1=10.5 mA
[0057] Fusion Time 2=0.2 sec
[0058] Fusion Current 2=17.5 mA;
[0059] Fusion Time 3=0 sec
[0060] Fusion Current 3=0 mA.
[0061] FIG. 11 shows a table 120 illustrating a heating current
profile used as setpoints to the power supply to obtain the results
set forth below. It is also possible to obtain good results by
manually adjusting the temperature of the heating tube 112, such as
by manually adjusting the current flowing through the heating wire
114, while splice loss is monitored during the heat treatment.
After the completion of the heating current profile shown in FIG.
11, the furnace is allowed to cool to ambient temperature for
approximately 1 min. The splice 80 and the DCF and BF fibers 50 and
60 are then removed from the furnace. The distal end of the BF
fiber 60 is now spliced to the SMF fiber 70, using a second set of
parameters, as described above.
[0062] An important parameter is the maximum temperature for the
heat treatment illustrated in FIG. 10. This parameter may be
determined empirically. A test splice 80 between a DCF fiber 50 and
a BF fiber 60 is mounted into the furnace 110 and splice loss is
monitored during heating. Several heating trials are now made, in
which the maximum heating current is varied (i.e., in which the
maximum temperature of the splice is varied) and in which the
cooling ramp is kept constant. The ramp can be linear, or may also
be non-linear, such as the ramp set forth in the table 120 shown in
FIG. 11. If the maximum current is increased for each new trial,
the same splice 80 can be used for the experiment. FIG. 12 shows a
graph 122 illustrating the trend of splice loss, after cooling, as
a function of maximum heating current. From this graph 122, it will
be seen that there is an optimal value of maximum current at which
the desired splice loss is obtained. After determining the maximum
current, the ramp can now be optimized by monitoring splice loss
during cooling.
[0063] FIG. 13 shows a table 124 containing data for splice loss
reduction by applying the above-described heat treatment to a
WideBand DCF sample manufactured at Lucent Technologies Denmark
A/S. The table 124 compares the amount of splice loss at a signal
wavelength of 1550 nm before and after heat treatment. At higher
wavelengths, the loss reduction is even greater. For example, at a
wavelength of approximately 1600 nm, splice loss is reduced several
dBs for some DCF designs.
[0064] According to a further aspect of the invention, the
above-described heat treatment of the splice between the DCF and BF
fibers is accomplished using a laser. The use of a laser, as
described below, allows the heat treatment to be performed without
making physical contact with the splice point. This is useful, for
example, in the construction of a high-strength optical
transmission line. Another advantage of using a laser is that a
very compact heating zone can be obtained. Thus, when the heating
zone is provided by a laser, a smaller length of bare fiber (for
example, 1 cm) may easily be used.
[0065] FIGS. 14A and 14B show, respectively, elevation and plan
views of a length of DCF fiber 130 and a length of BF fiber 132
that have been spliced together at a splice point 134 and mounted
into a frame 136. FIG. 15 shows a diagram of a laser heating system
140, in which the frame-mounted, spliced DCF and BF fibers 130 and
132 shown in FIGS. 14A and 14B are positioned proximate to a
C0.sub.2 laser 142, or other laser of suitable power, such that the
laser beam 144 emanating from the laser 142 provides the required
heating of the splice point 134. The temperature of the splice
point 134 is regulated by adjusting the power of the laser beam
144. According to a further aspect of the invention, the spliced
fibers 130 and 132 and the frame 136 are housed in a chamber 146
that is filled with a protectant gas, such as nitrogen.
Alternatively, the splice point may be purged with a stream of
protectant gas during the heat treatment. In this case, the heat
treatment may be conducted in a normal ambient atmospheric
environment.
[0066] It should be noted that the graphical technique illustrated
in FIG. 12, described above, for empirically determining an optimal
maximum temperature for heating the splice between the DCF and BF
fibers can also be used in connection with a laser-heated system,
such as the system 140 illustrated in FIG. 15. However, instead of
graphing splice loss as a function of maximum heating current,
splice loss is instead graphed as a function of maximum laser beam
intensity. In all other significant respects, the technique is the
same.
[0067] According to a further aspect of the invention, splice loss
is reduced as part of the splicing operation. Specifically, a
fusion splicer is used to splice together a first and a second
optical fiber. After the fibers have been spliced together, the
temperature of the splice point is ramped downward from the
splicing temperature to a cooler temperature by ramping down the
arc current used to drive the fusion splicer's electric arc.
[0068] FIG. 16 shows a diagram of the main components of a fusion
splicer 150 according to the present invention. The fusion splicer
150 includes a chassis 152, upon which is mounted a pair of optical
fiber routing guides 154 and 156. The optical fiber routing guides
154 and 156 are used to hold a pair of optical fibers 158 and 160
that are to be spliced together at a splice point 162.
[0069] The splice point 162 is positioned to lie in the path of an
electric arc 164 that is generated between two electrodes 166 and
168. A variable current source 170 is used to provide power to the
electrodes 166 and 168. The intensity of the electric arc 164 and
the amount of heat that is delivered to the splice point 162 is a
function of two parameters: the level of the current delivered by
the current source 168 to the electric arc 164, and the amount of
time that the splice point 162 is exposed to the electric arc 164.
Suitable parameters are chosen based upon the particular fibers
being spliced together.
[0070] As shown in FIG. 16, the fusion splicer 150 is provided with
a controller unit 172 connected to the current source 170 that
allows the level and duration of current delivered by the current
source 170 to the arc electrodes 166 and 168 to be controlled
automatically. The controller unit 172 may suitably be driven by a
microprocessor or other suitable programmable device. It should be
noted, however, that once suitable time and current parameters have
been determined, it would also be possible for the arc current
controller 172 to have fixed settings. It should further be noted
that the present aspect of the invention can also be practiced
using currently available fusion splicers, by manually adjusting
the arc current to achieve the desired ramping.
[0071] According to the prior art, once the two fibers 158 and 160
have become fused at the splice point 162, the electric arc 164 is
typically cut off abruptly, resulting in a precipitous drop in
temperature. However, according to the present aspect of the
invention, after the two fibers 158 and 160 have become fused at
the splice point 162, the current from the variable current source
170 is instead ramped downward in a controlled manner, thereby
producing a controlled ramping down of the temperature at the
splice point 162 from the splicing temperature down to a cooler
temperature. By ramping the temperature of the splice downward in a
controlled manner, it is possible to achieve a reduction in splice
loss similar to that achieved by using the other techniques
described herein.
[0072] The parameters used to construct a suitable arc current ramp
are determined empirically, using techniques similar to those
described above in conjunction with FIGS. 11 and 12. It should be
noted that, because of the intense heat generated by a fusion
splicer, it may be difficult to create a controlled, gradual
ramping of the temperature of the splice from the splicing
temperature all the way down to room temperature. However, even
ramping at higher temperatures will result in a reduction of splice
loss.
[0073] It will be appreciated that, according to this aspect of the
invention, the ramped cooling of the splice is now a continuous
part of the splicing process. Thus, this aspect of the invention
may be more suitable for certain applications, such as
high-strength applications, as no additional treatment is required
after splicing. This aspect of the invention may suitably be used,
for example, with the new Inverse Dispersion Fibers (IDF) recently
introduced by Lucent Technologies.
[0074] FIGS. 17 and 18 are flowcharts of a first method 180 and a
second method 200 for reducing splice loss in an optical
transmission line according to aspects of the present invention.
These methods 180 and 200 may suitably be used in conjunction with
the optical transmission line 40 shown in FIG. 4, and discussed
above. However, these methods 180 and 200 may also suitably be
employed with other combinations of optical fibers. For example,
these methods may be applied in splicing together two lengths of
Inverse Dispersion Fiber (IDF.times.IDF).
[0075] In the first method 180, two optical fibers are spliced
together in step 182. In step 184, the spliced fibers are removed
from the splicer. In step 186, the splice is then heated to a
predetermined maximum temperature. As described above, the splice
may suitably be heated using a furnace, such as that shown in FIG.
10, or a laser, such as that shown in FIG. 15. In step 188, the
splice is then cooled using a predetermined ramp. The shape of the
ramp may suitably be determined using the empirical techniques
described above.
[0076] In the second method 200, two fibers are spliced together in
step 202. As described above, the fibers may suitably be spliced
together using a fusion splicer. In step 204, the arc current of
the fusion splicer is then ramped downward to achieve a controlled
ramp of the temperature of the splice down from the splicing
temperature to a cooler temperature. As described above, this
ramping of the temperature of the splice results in a reduction of
splice loss. In step 206, the spliced fibers are removed from the
splicer.
[0077] In an optical transmission line with multiple splices, it
would be possible to perform the first method 180 or the second
method 200 on more than one splice. Further, it would also be
possible to perform the first method 180 on a first splice in the
optical transmission line and the second method 200 on a second
splice in the optical transmission line. Where the second method
200 is performed on multiple splices in an optical transmission
line, the arc current is adjusted between splices in preparation
for the next splice. As mentioned above, depending upon the
particular fibers being spliced together, the splicing parameters
may vary from splice to splice.
[0078] While the foregoing description includes details which will
enable those skilled in the art to practice the invention, it
should be recognized that the description is illustrative in nature
and that many modifications and variations thereof will be apparent
to those skilled in the art having the benefit of these teachings.
It is accordingly intended that the invention herein be defined
solely by the claims appended hereto and that the claims be
interpreted as broadly as permitted by the prior art.
* * * * *