Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention relates to fiber optic waveguides. One
advantage of a single mode optical waveguideJ and a restricted -~
mode guide, over a multimode guide is the elimination or reduc-
tion of mode dispersion. A disadvantage is that, for a given
core to cladding refractive index difference, the core diameter
is smaller than that of a corresponding multimode fiber. A
small core presents particular problems in the making of perman-
ent and demountable butt joints between fibers because a lateral
displacement of the axis of one fiber with respect to that of
the other gives rise to a coupling loss that inGreases with a
reduction in core size. The core diameter can be increased so
as to reduce this misalignment problem, but this requires a
reduction in the core to cladding refractive index difference.
One result is that the mode or modes are less tightly bound to
the core, and hence radiative losses at bends in the fiber are
increased. j`
According to one aspect of the present invention, there is
provided an optical fiber comprising a core having a central region of
constant material composition and thickness terminated at both ends by
end regions of varying composition and thickness to provide a normalized
optical frequency having the same single bound non-radiating mode in
both the central and terminal regions.
According to another aspect of the invention there is provided
in a method of fabricating a limited mode optical fiber by forming an
optical fiber preform including a cladding layer and a core layer deposited
within said cladding layer, collapsing the preform and drawing the preform
into an optical fiber, the improvement comprising the steps of: depositing
core forming materials along the length of the preform; maintaining a
substantially constant thickness and composition of the deposited core
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forming materials along the length of a central core region; increasing the :
thickness of the deposited core forming materials along the lengths of
graded core end regions so that the deposited core forming materials become
thicker as the ends of the preform are approached and the preform ends have
enlarged core thicknesses; and changing the composition of the deposited
core forming materials along the lengths of the graded core end regions in ~ ,
accordance with the increased thickness of the deposited core forming
materials along the lengths of the graded core end regions so that the :
preform will provide a fiber having a substantially constant normalized
frequency throughout the length of the optical fiber.
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A fiber produced by this method can be designed to combine
the advantage of a large cored small refractîve index difference
fiber so far as coupling efficiently between butt jointed fibers
is concerned, while benefiting, over the length of the central
portion, which may be very long compared with the graded portion,
from the relatively small radiative bending losses of a small
cored large refractive index difference fiber.
The refractive index profile of fiber produced by this
method may be step-index, graded index, such as a parabolic
profile, or a more complex structure, such as the W-guide or ~-
O-guide profiles. In all cases the core to cladding refractive
index difference for the central region will normally be com-
paratively large, being typically about 1%, with the result that
the transmitted energy is relatively tightly coupled to the core
region within this portion. Considering, for illustrative
purposes a step index single mode fiber, the normalized frequency
(V) is given by:
V = 2~a ~n12_n22)l/2 ~-1
where 2a = core diameter
~ = guided wavelength -
nl = core refractive index
n2 = cladding refractive index ;~
Substituting n = 1/2~nl~n2)
and ~n nl n2
gives V = 4~anl/2~nl/2-~
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Then as ~ and n will remain virtually constant~
V ~ a ~nl/2
Thus a parabolic relationship between the core diameter and
refractive index difference is required to maintain a cons-tant ;
normalized frequency. Thus, for instance, the core thickness is to be
increased by an order of magnitude from about 3~ at the central
core region to about 30~ at the ends of the graded core end regions, the
refractive index difference must decrease in a corresponding
manner by two orders of magnitude, that is from a difference of
about 1% to about 0.01%. In principle the grading can be over a -
distance of no more than a few centimeters, but, in practice,
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since the fiber is drawn from a preform, and this normally involves
an extension of at least a thousand-fold, the grading is longer,
and typically extends over several meters or even several tens of
meters. Within the graded core end regions the fiber will have the
sensitivity to bending losses appropriate to the core size in -
those regions, but since these graded core end regions will normally
represent a very small proportion of the total length of the
fiber, the overall sensitivity to bending losses of the fiber
considered as a whole will approach the lower limit set by the
size of the core in the central core region.
Brief Description of the Drawing
Figure 1 is a side view of the apparatus employed in form-
ing the core material of the instant invention.
Description of the Preferred Embodiment
A thermally induced chemical vapor reaction is used for the `~
creation of core material for the fiber because such a reaction
is relatively readily controllable, both in the rate of glass
formation and in its composition. The reaction may be a hydroly-
sis reaction, but an oxidation reaction from which hydrogen and
hydrogen containing compounds are excluded is preferred because
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the latter reaction does not produce -OH groups in the final
product. Such groups are normally undesirable on account of the
position of their optical ahsorption bands in the near infrared
region of the spectrum.
The core material may be built up by the reaction on a bait
to produce a self-supporting rod of core material which is then
coated with a lower refractive index cladding glass by a similar
reaction. Alternatively the core material may be deposited as a
layer upon the surface of a rod which is removed after the depo-
sition of a subsequent layer of lower refractive index cladding
glass upon the core material. The bore of the resulting hollow
structure may then be collapsed as an additional process step
prior to the drawing of the preform into fiber. our preferred
method of creating the core ~aterial is however to deposit the
core material upon the bore of a tube of lower refractive index
cladding glass. This tube of cladding glass is not necessarily
a self-supporting structure, but may itself be a lining deposited
upon the bore of a substrate tube. This use of a lining enables
the use of a substrate tube of lower optical quality than would
be required if the tube were contiguous with the core, and thus
penetrated by a significant proportion of propagating optical
energy.
Fig. 1 shows a substrate tube 1 of fused silica having,
typically, an external diameter of 10 mm and a wall thickness of
1 mm mounted in a modified lathe ~not shown). The ends of the
tube 1 are mounted in rotary seals 2 and 3 connected respectively
to an inlet pipe 4 and an exhaust pipe 5. The modified lathe is
provided with a pair of chucks which are driven at the same
speed so that when a short length of the tube is softened by heat,
the softened region i6 not required to transmit torque from the
portion on one side of the region to the portion on the other.
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The inlet pipe 4 is coupled to vapor blending apparatus ~not
shown~ in which a facility is provided for entraining vapors
from volatile liquids in individual gas streams which are mixed
prior to their delivery from the apparatus into the inlet pipe 4.
Control of the composition is determined by flow rates of the
respective gas streams and by control of the temperatures of the
various volatile liquids. The tube 1 threads a short heater 6
which can heat a short section of about 2 cm of the tube. This
heater can be translated up and down substantially the whole
length of the tube. Conveniently the heater takes the form of a
gas burner having a set of geometrically arranged inwardly
directed flame jets.
First a layer of uniform thickness of silica is deposited
upon the bore of the tube to provide the lower refractive index
cladding material of the completed fiber. This deposition is
followed by the deposition of the core material whose composition
and thickness varies along the length of the tube. The core
material is required to have a refractive index that is greater
than that of the cladding glass, and this is provided by co-
depositing silica with a suitable dopant. Many suitable dopants
are possible, including oxides of germanium, arsenic, antimony,
indium, gallium, phosphorus and aluminum, used either individu-
ally or in selected combination. The silica and the dopants are
conveniently deposited by reacting their halides or oxy-halides
with oxygen.
The vapor blending apparatus is first used to producing the
cladding material by bubbling a stream of dry oxygen gas through
silicon tetrachloride maintained at 0C, and this is diluted with a second ~,
gas stream of pure dry oxygen gas. The resulting entrained silicon tetra-
chloride does not react with the oxygen at room temperature but
requires a temperature in the region of 1200C. Thus the vapor
reaction proceeds only in the localized region of the tube
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heated by the burner 6. At about 1200C the reaction produces a
powdery deposit which has to be further heated to form a vitreous ;:
layer. For this reason it is preferred to use a flame which ~
take the heated region of the tube to a temperature about 1500C at ^
which the silica is deposited directly as a vitreous layer rather
than a particulate one. The burner 6 is traversed at a controlled
uniform rate along substantially the whole length of the tube so as
to build up a cladding layer of uniform thickness and composition.
Normally several traverses are required to build up a layer several
microns thick.
Using germania as a dopant, the process is then repeated
for the deposition of the core material, only in this instance a
co-deposition of silica and germania is required, and for this
purpose the vapor blending apparatus is adjusted to provide a
third gas stream of dry oxygen, this being bubbled through
germanium tetrachloride maintained at 0C. Another difference is :
that the flow rate of the gas stream through the silicon tetra- ` -
chloride is not maintained constant, but is varied as a function of
position of the burner 6 along the length of the tube. The flow -~`~
rates through the silicon tetrachloride and through the germanium
tetrachloride are held constant along the central core region of the
length of the tube, which is typically a meter or more long, but for -
the first and last 1 to 3 cm of each traverse, they are independently
varied. At the beginning of a traverse the flow rate of the gas
stream through the silicon tetrachloride is reduced substantially
linearly, while at the same time the flow through the germanium
tetrachloride is increased according to a quadratic function. Near the
end of the traverse the flow rates are adjusted in the same manner, but
in the opposite sense, with the flow through the silicon tetrachloride
being linearly increased, while that through the germanium tetrachloride
is
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quadratically reduced. As before, several traverses will
be required to build up the requisite thicknesses of core
material.
Thus by varying the flow rate of silicon tetrachloride
along the length of the tube the thickness of the core is
changed. The central core region has a constant thickness while
the graded core end regions increase in thickness as the ends
of the tube are approached. The variation in the flow rate of
the germanium tetrachloride causes a variation in core composi-
tion since it provides the dopant. Thus, the central coreregion has a constant composition while the graded core end
regions have a changing composition with decreasing dopant and
decreasing refractive index as the ends of the tube are ap-
proached. ~;
In the above described embodiment, the variation in the
core thickness and composition is realized by variation of
the flow rates of the core forming materials while holding the
temperature and the burner traversal rate constant. Other
methods of varying the core thickness and composition will be
discussed subsequently.
In a modification of the above described deposition methodthe cladding glass layer is not of pure silica, but lS of
doped silica. An adYantage of using a doped silica for the ;
cladding glass layer is that the presence of a dopant in the ~'
reaction tends to reduce the temperature at which the deposit
comes down as a vitreous layer. Conveniently the same dopant
is used for both deposited layers, the dopant concentration be-
ing greater in the case of the core material layer.
A further modification also uses a doped silica cladding
glass layer, but in this instance the dopant is boric oxide.
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Boric oxide is unlike other dopants in that it can be used ;
; to reduce the refractive index of silica. It is thus possible
to construct a preform where cladding material is doped with
; boric oxide to a greater extent than the core material, and
where the doping level of the core increases where the core
diameter is expanding.
In each of these instances, once the deposition of the
two layers has been completed, the burner is adjusted to give
an increased output sufficient to raise the temperature of the
heated region of the tube to its softening point. The burner
is then traversed along the tube a final time causing its
bore to collapse. The balanced drive to both ends of the
tube rernoves any tendency for the tube to twist up during this `
collapsing process. Finally, the collapsed tube is withdrawn
from the lathe, mounted in drawing apparatus (not shown), and ; ~`
drawn at a constant rate into fiber. The extension produced
by the drawing is chosen having regard to the composition and ~
size garding of the core material of the praform to produce a ; -
single mode fiber with a substantially constant normalized
frequency lying in the range 2.0 to 2.~.
In the above described methods of forming the core
material of the preform the temperature of deposition and the
rate of relative movement between the burner and the tube were
kept constant while only the flow rates of the gas streams
were varied. However since both the temperature and the rate
of relative motion affect the deposition rate, the refractive
index and thickness of the deposit may be controlled by suit-
able manipulation involving the variation of either or both of
these two parameters. The particular way in which the varia- `
tion in core thickness and composition is realized is not
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critical to the present invention. What is important is
that the core thickness be increased as the ends of the
preform are approached and that the core eomposition be
varied in relation to the thickness so as to maintain a sub-
stantially eonstant normaliæed frequeney.
It is to be understood that the foregoing deseription of
speeifie examples of this invention is made by way of example
only and is not to be eonsidered as a limitation on its seope.
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