Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOW DISPERSION SINGLE MODE FIBER
Background of the Invention
l. Field of the Invention
This invention is a low-loss (less than 1 dB/km
at 1.30 ~m) single mode fiber with low dispersion (less
than 5 psec/nm-km) within the wavelength range 1.25-l.385 ~m
and having low added loss (less than .25 dB/km) due to cabling.
2. Disclosures of Interest
Full appreciation of the advances represented by
the inventive fiber requires at least a cursory review of
certain aspects of fiber design technology.
The realization of low-loss optical fibers in the
early 1970's focused research on the attainment of higher
bandwidth for greater information carrying capacity.
Initially, graded multimode fibers were pursued, in part,
because they were easier to fabricate than single mode
fibers. However, workers were always aware that single
mode fibers have greater inherent potential for high
bandwidth, and as years passed, the search for ever higher
bandwidth fibers once again focused attention on single
mode fibers.
It was known that although single mode fibers display
none of the inter-mode dispersion associated with multi-
mode fibers, they do have finite pulse spreading, andhence bandwidth limitation, due, in part, to material
dispersion - the dependence of index of refraction, and
consequently traversal time, on wavelength. Any pulse, by
Fourier definition a combination of many different wave~
lengths, will therefore experience broadening when
traversing the fiber. ~owever, the material dispersion
phenomenon does vanish at certain wavelengths - e.g.,
approximately 1.27 ~m for fused silica, 1.35 ~m for heavily
doped germania silica, and 1.25 ~m for fluorine-doped
silica - and consequently these might appear a~ first sight
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to be desirable operating waveleng-ths, all other
considerations being equal. Nevertheless, it was found
that even at the material dispersion null point relatively
significant pulse broadening did occur due, in part, to
waveguide dispersion - the wavelength dependence of
traversal time associated with purely waveguide parameters.
First principles indicate that in certain regions
of the spectrum disper.sive effects associated with
waveguide dispersion are of opposite sign than those
associated with material dis~ersion. Consequently, the
possibility arises that fibers may be designed with a view
toward cancelling material dispersion against waveg~ide
dispersion and hence yielding essentially zero dispersion
at a particular wavelength (H. Tsuchiya et al, Electronics
Letters, 15, 476 (1979)). Desirable wavelengths for
predetermined zero dispersion include 1.55 ~m where the
loss properties of a silica-based fiber are lowest. lIn
"W-type" fibers it was found that low dispersion could be
obtained over a relatively broad wavelength range,
(K. Okamoto et al, Electronics Letters, 15, 729 (1979)).]
In order to obtain sufficient waveguide dispersion
to cancel the material dispersion at 1.55 ~m in typical
germania doped single mode fibers, relatively small core
diameters must be used, since waveguide dispersion
increases in magnitude with decreasing core diameter. The
use of a graded core may permit a somewhat larger core
diameter, however, the effect of core diameter on splicing
always remains a serious consideration which must be
carefully weighed in the design of high bandwidth single
mode fibers. Furthermore, even if small core single mode
fibers for operation at 1.55 ~m would be feasible, they
would be relatively useless at the present time since there
is a dearth of high quality commercially available,
spectrally narrow, light sources operating at 1.55 ~m. This
has forced the worker in the field to focus on other
spectral regions where sources are available and where
local minima in transmission loss occur. Such a region
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where commercial sources are available and where there is a
local minimum in loss, occurs in the vicinity of 1.3 ~m,
(1.25-1.385 ~m), stimulating interest in single mode fibers
for operation in this spectral region.
A threshold consideration for operation at shorter
wavelengths, such as 1.3 ~m, involves the need to lower
the cutoff wavelength ~ to values close to, but below,
the operating wavelength. The cutoff wavelength is
that wavelength below which higher order modes may be
propagated. Most desirable transmission characteristics
occur when the transmission wavelength is somewhat above,
but close to, cutoff. Operation at 1.5 ~m allows
relatively high cutoff wavelengths, i.e., approximately
1.45 ~m. However, single mode operation at 1.3 ~m requires
much lower cutoff wavelengths.
The cutoff wavelength is proportional to the
product of the core diameter and the square root of ~,
where ~ is the relative index difference between the core
and the cladding. Hence, for low cutoff wavelengths this
product must be small. However, ~ itself must be
relatively small in typical single-mode fibers since in
high A fibers the material dispersion, a quantity that
generally increases with increasing A's, would be too high
to allow cancellation by waveguide dispersion at 1.3 ~m.
This is so since the waveguide dispersion at 1.3 ~m is
larye enough to cancel material dispersion in high Q fibers
only if the core diameter is extremely small. It would
consequently appear that low dispersion (high bandwidth)
single mode fibers for operation at 1.3 ~m would require
relatively small values of ~. However, if ~ is too small~
packaging losses become too high. A satisfactory design
for high bandwidth low packaging loss single mode fibers
for operation in the vicinity of 1.3 ~m has consequently
eluded workers in this field.
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Summary of the Invention
According to the present invention there is
provided a single mode fiber fabricated from a substrate
tube comprising a) an up-doped core which contributes a
refractive index increase relative to that part of the
fiber originating in the substrate tube of less than .40
percent and greater than .30 percent, b) a down-doped
cladding with no abrupt change in index of refraction
which contributes a refractive index decrease relative
to that part of the fiber originating in the substrate
tube of less than .22 percent and greater than .15 per-
cent, c) a core diameter of between 8.7 and 7O5 microns
and, d) a ratio of diameter of the down-doped cladding
to the diameter of the up-doped core of greater than 5.5,
and a cut off wavelength less than 1.31 ~m, the ~ of the
fiber being greater than .3 percent and less than .75
percent, the dispersion of the fiber being less than 5
psec/nm-km within the wavelength range of 1.25-1.385 ~m.
This invention is a low-loss [less than 1 dB/km)
single mode fiber with low dispersion (less than
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5 psec/nm-km) within the wavelength range 1.25-1.3~5 ~m,
and having low added loss (less than .25 dB/km) due to
cabling. The fiber has relatively high A (greater than
.3 percent) to assure low cabling loss. The high value of
~ is obtained without paying a cost in high material
dispersion by providing at least 20 percent of the ~ by
down-doping of the fiber cladding (adding a dopant to lower
the index of the material). The resulting relatively small
amount of material dispersion is cancelled by an
appropriate amount of waveguide dispersion so as to obtain
low dispersion values in the vicinity of 1.3 ~m
(1.25-1.385 ~m). Although relatively small core diameters
(less than 9 ~m) are required for appropriate waveguide
dispersion values, splicing losses are acceptable due to
low contributions from angular offset at the splice when
transverse offset is significant.
Brief Description of the Drawing
FIG. 1 is a schematic representation of the inventive
fiber.
FIG. 2 is a representation of the refractive index
configuration of an embodiment of this invention.
Detailed Description
The problem addressed by this inven-tion becomes
one of how to obtain values of ~ and core diameters in a
low dispersion, low-loss, single mode fiber which will
provide cutoff ak sufficiently low wavelengths for
opexation at the local loss minimum which occurs in the
vicinity of 1~3 ~m. A solution to this problem, as
manifested in this invention, relies on the realization
that an up-doped core with a down-doped cladding can, at
once, provide a high A and low material dispersion in the
vicinity of 1.3 ~m. "Down dopants", such as fluorine, in
the cladding combine with the up doped core to yield low
material dispersion. The effect of the cladding in
yielding low material dispersion values will be significant
since in single mode fibers large amounts of energy
propagate within the cladding. ~onsequently, an inventive
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aspect of the fiber claimed in this application is tied to
the realization that high Q does not necessarily result in
high material dispersion in the vicinity of 1.30 ~m, when
reliance is had on a do~n-doped cladding. Sufficiently low
cutoff wavelengths are obtained by employing relatively
small core diameters with the relatively high ~'s which
guarantee low packaging loss.
The inventive fiber does require relatively small
core diameters (less ~han 9 ~m), and practitioners have
been hesitant to use such small core single mode fibers for
fear of prohibitive splicing losses. However, theoretical
studies [D. Marcuse, Bell System Technica~ Journal, _, 703
(1977)] indicate that the product of splicing loss due to
transverse offset and angular offset is approximately
constant, hence allowing consideration of small core
diameter fibers. Although such a fiber may suffer
significant splicing loss due to transverse offset of the
splice, the fiber will have lo-~ splice loss due to angular
offset, and vice versa, hence rendering the splicing loss
problem somewhat less serious than had been widely
considered previously.
While the inventive fiber is patentably distinct
merely on the basis of its design characteristics, the
motivating factors which result in these design
characteristics heighten, still further, the patentable
aspects of the subject fiber.
Whereas previously, the practltioner who
attempted to obtain zero total dispersion engineered the
waveguide dispersion to cancel the material dispersion, the
designer of the subject fibers approaches his task from a
totally different vantage point. The subject fibers are
designed in the first instance by specifying a ~ which is
sufficiently high so as to obtain a desirably small spot
size. The spot size is inversely proportional to the
square root of Q and if ~ is high enough,the spot size is
small enough to yield a desirably low cabling loss. ~'s in
these fibers ~re yenerally greater than .3 percent yielding
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spot sizes generally less than 4 ~m. (The spot size here
is defined as the fiber radius at the 1 power point).
The next step in the design is to determine an
appropriate cutoff wavelength, depending on desired
operating parameters, and setting the core diameter of the
fiber accordlngly. In the instant fibers, the cutoff
wavelength is set at approximately 1.25 ~m (1.20 ~ 0.1 ~m)
in view of the fact that the operating wavelength is
contemplated to be at 1.31 ~m. Required core diameters are
then less than 9 ~m.
Having determined the A and the core diameter of
the fiber, the waveguide dispersion of the fiber is
essentially fixed and cannot be effectively used to
determine a zero dispersion wavelength, as in the prior
art. However, in a departure from the prior art, applicants
alter the material system used to fabricate the fiber
so as to obtain a material dispersion value which will
Cancel the waveguide dispersion in the vicinity of 1.31 ~m.
The demands on the material system are then both the
requirement of relatively high ~, as previously discussed,
and relatively low material dispersion to cancel the
waveguide dispersion. In typical germania-doped single
mode fibers, high Q's result in relatively high material
dispersion. As discussed above, in the inventive fiber the
high Q is obtained, in part, by down-dopin~ -the cladding to
obtain a high ~ while at the same time obtaining a
relatively low material dispersion. FIG. 1 is then a
schematic representation of the inventive fiber 11, with
up doped core 13 and down~doped cladding 12. A portion of
the fiber associated with the substrate tubes used in MCVD
are not necessarily shown.
Other considerations also demand that both the
cladding and the core compositions be available as variable
parameters at this point in the design. If the cladding of
the fiber is somehow predetermined, then the only remaining
parame~er which might affect the material dispersion is the
composition Gf the core. However, if the claddiny of the
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fiber were prede~ermined, the previous fixing of a would
determine the index of refraction of the core as well.
Consequently, in the inventive fibers the composition of
both the cladding and the core are left as variable
parameters at this point in the design.
Both the desire to obtain zero dispersion in the
vicinity of 1.31 ~m, and the desire to obtain a relatively
low loss fiber, results in the selection of a lightly up-
doped core in the subject fiber. Consequently, in the
inventive fiber the core is doped with, for example,
germania to a level less than 5 mole percent. However, to
obtain the necessary predetermined ~ and at the same time a
relatively low material dispersion, the cladding must be
deeply down-doped with a material which lowers the index of
refraction of the cladding much below that of the core.
Boron which is known to have this capability has a strong
absorption band at 1.3 ~m and, hence, is undesirable.
However, fluorine which also tends to lower the index of
refraction has a higher wavelength absorption band and
consequently may be used in the inventive fiber to down-
dope the cladding.
The index distribution in an embodiment of -the
inventive fiber is then shown in FIG. 2. In this FIGUE~,
25 is the up-doped core region of the fiber and 26 is the
down-doped cladding region. That portion of the ~ of the
fiber which is attributable to the down-doped cladding is
shown schematically as 22 and accounts for at least
20 percent of the fiber ~, 23. The remainder of the fiber
~, 24 is clearly due to the up-doped core. 21 is the index
value of the substrate tube and in many embodiments will be
essentially pure silica. ~owever, other inventive
embodiments may involve doped substrate tubes, in which
case the index of refraction of the substrate tube shown as
21 may be equal to that of the cladding 26.
Characteristics of the Inventive Fiber
1. Mode Characteristics
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The fiber is a single mode op-tical fiber.
Clearly, such a definition has meaning only in the context
of a particular transmission wavelength. Any fiber will
support more than one mode at low enough wavelengths~ In
order for it to be a single mode fiber, the fiber must be
operated in a region a~ove the cutoff wavelength. The
inventive fiber will have a cutoff wavelength less than
1.31 ~m and will be a single mode fiber for transmission
wavelengths above the cutoff value. The term "single mode
fiber" is used to indicate operation in such a region. In
any event, the fiber will be clearly distinguished from
multimode fibers which suppor-t many hundreds of modes as
opposed to a single mode fiber which even below its cutoff
wavelength supports only a limited number of modes in the
region of the spectrum from .4 to 2 ~m~
2. ~ Value
The inventive fiber is , in part, characterized by
relatively high ~ values, e.g., grea-ter than .3 percent,
although less than .75 percent. Definitions of ~ vary from
practitioner to practitioner. In the current context ~ is
defined as the index of refraction of the core minus the
index of the cladding all divided by the index of
refractions of the cladding. High ~ values are obtained in
the inventive fiber, without paying a material dispersion
penalty, by down-doping the cladding in a silica based
fiber with material such as fluorine to yield a fiber that
has a lower zero material dispersion point than fibers
which are solely up doped. In the inventive fiber at least
20 percent of the ~ value will be attributable to the
down-doping of the cladding. Recent studies indicate that
graded index single mode fibers may have desirable
characteristics at least, in part, in allowing larger
diameter cores. Clearly, the inventive fiber contemplates
possible use of such a gradation in the index of
refraction. Under such a circumstance, ~ is defined by the
associated maximum index of refraction of -the core and
minimum index of refraction of the cladding.
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3. Core Diameter
The core diameters in the inventive fibers are
determined from a design point of view by requiring the
cutoff wavelength to he below the operating wavelength of
approximately 1.30 ~m. The waveguide dispersion is then
uniquely defined and must be cancelled by appropriate
values of material dispersion. Such cancellation re~ults
in an essentially zero ~otal dispersion within the
operating wavelength of interest, namely, 1.25 to 1.385 ~m.
Core diameters in this fiber design, necessary for such low
total dispersion, are less than 9 ~m, a departure, at least
from currently preferred practice. This departure can be
tolerated even in the face of splicing considerations due
to applicants' appreciation for the inverse behavior
between splicing loss due to angular offset and that due to
transverse offset.
4. Cladding to Core Ratio
Disclosures currently available discuss "W-type
fibers" in which the cladding is down-doped. ~uch fibers
generally have claddings which comprise two specific
regions separated by an abrupt change (generally greater
than .0038) in the index of refraction. However, the
inventive fiber described in this application generally has
no such abrupt change in the i.ndex distribution of the
cladding and, in addition, generally has a down-doped-
cladding to core diameter ratio greater than 2, thereby
clearly distinguishing it from the down-doped fibers
(including W-type) currently described in the literature.
Of course, the subs-trate tube which may be used in
fabricating the inventive fibers might have an index of
refraction higher than the cladding giving the appearance
of a W-type configuration, namely, an up-doped core, a
down-doped cladding region and a second outer higher index
of refraction region. However, the requirement that the
inventive fibers have no abrupt index change in the
cladding and have a down-doped-cladding to core ratio
greater than 2 is meant to avoid essentially all W-type
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fibers.
Other inventive embodiments involve use of a doped
substrate tube. In such circumstances, the index of
refraction of the substrate tube may equal that of the
cladding yielding only a single index of refraction region
from cladding to substrate tube region with no abrupt
change in the index.
5. Dopants
At the present time preferred dopants involve
germania in the core and fluorine in the cladding.
Clearly, the inventive fiber need not be limited to these
specific dopants. However, when they are used it is found
that the core will generally be up-doped with less than
5 mole percent germania and the cladding will generally be
down-doped with greater than .5 mole percent fluorine. The
addition of other dopants, such as phosphorus, for example
in the cladding, in part, for improved processing
characteristics, may be contemplated within the spirit of
this invention.
Example
1. Fiber Fabrication
The preform was made by MCVD
(U.S. Patent 4,217,027) using a l9x25 TO8-WG silica tube.
The reactant flow rates for cladding deposition were SiC14
3.0 gm/min.; POC13 0.052 gm/min.; CF2C12 105 cc/min~ and
excess oxyyen 4300 cc/min. The cladding was deposited in
16 passes. No pressurizing device was used since the tube
shrinkage amounted to only about 1 mm in the OD over the
course of the deposition. The core was deposited in 2
passes using flows of 0.54 gm/min. SiO2, 0.077 gm/min.
GeC14 and 1300 cc/min. excess oxygen. Compensated collapse
was accomplished in 6 shrinking passes during which a trace
of GeC14 vapor carried on oxygen was flowed through the
tube. The tube was then sealed at the downstream end and
collapse was completed in 2 more passes.
After measuring the cross sectional dimensions of
the preform in an immersion cell, fiber was drawn and
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coated in-line with a UV cured epoxy acrylate resin. The
fiber dimensions were OD 114 ~m; core diameter 7.5 ~m, D/d
(cladding-to-core diameter ratio) 5.9, and length 1 km.
2. Characterization _ Fiber
The fiber was characterized by measuring the
cutoff wavelength, the spectral loss and the total
dispersion.
Cutoff was determined as the location of the
rapid drop in power transmitted through a 3 meter length as
the wavelength of the inciden-t light was increased. ~
well-defined cutoff was located at ~ = 1.192 ~ .005 ~m.
The spectral loss was measured from 1.0 to 1.7 ~m
using a far end/near end technique with a 3 meter near end
length. The loss is measured with and without a single
40 mm radius loop in the near end length. Quite
surprisingly, the loss curve without the loop was
essentially identical to that with the loop, even in the
vicinity of cutoff at 1.19 ~m. It has been our experience
that this is a signature of very good mode confinement.
The loss has a local minimum at 1.30 ~m of
0.57 ~ .03 dB/km, a local maximum at the 1.39 ~m O~ peak of
7.7 dB/km, and a minimum loss of 0.40 dB/km at 1.50 ~m.
Beyond 1.50 ~m the loss rises rapidl~ and all evidence
indicates tha-t the loss is unbounded. The loss at 1.30 ~m
is not adversely affected.
The total chromatic dispersion in the single mode
regime was calculated from the derivative of group delay
versus wavelength data. This data was obtained using
narrow pulses from a Raman fiber laser pumped with mode-
locked Q-switched pulses at 1.06 ~Im from a Nd-YAG laser.
The wavelength of the pulses emerging from the Raman fiber
was selected with a grating monochromator. The zero
dispersion wavelength, ~O is 1.314 ~m.
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I
3- Ex~e_l____al ParRmetric S_ud~
aubseguant to the fabrication of the fiber
discu~sed in Se~ti~ns 1 and 2 of thi, example, numerous
fibers were fabric~ted with parameters ~hich varied about
~ho,e of the fiber described in Sections 1 and 2 of thi~
exampleO As a result of this P~rametric study, sPecific
ranges of design parameter~ for the preferred practice ~f
this invention were obtained~ ~hese preferred ranges
include an up-d~ed coce which _ontributes 3- refra-tive
index increase relativa to that Part of the fiber
originatiQg in the ~ubstrate tube of leas than .40 percent
and greater than .30 percent, a down-doped cladding which
_ontribut2s a refr~ctive in~ex decrease relative to that
part of the fiber originating in the substr~te tube of less
than .22 percent and greater th~n .15 percent, a core
diameter of between 8.7 and 7.5 microns and a ratio of
diameter ~f the do~n~do2ed _lad~in~ to the diameter of the
up-doped core 3f greater th~n 5.5. A specific emb~diment
found most preferable for commercial applicltion comprises
a fiber with an up-doped c~ce w1ose incre~,a in refcactive
in-lex relative to that Part of the fiber originating in the
- ~ubstca~e tube is .38 percent plus ~r minu~ .02 percent, a
down-dopeù cladding whose refractive index decraase
rel~tive to th~t part of the fiber originating i~ the
substrate tube is .19 plus or minus .02 per_ent, the
~iameter of the up-doped core being 8.3 plu, or minus
.2 microns and the ratio between the di~meter of the down-
doped cladding ~nd the dia~eter of the up-doped core being
6.0 plus or minus 0.2. In ~ f the above instances the
refra-tive index differences were measured using the
refracted neac field techni~ue 3n the fiber r~ther than on
the preform for greater accuracy.
