CA1061617A - Limited mode optical fiber - Google Patents
Limited mode optical fiberInfo
- Publication number
- CA1061617A CA1061617A CA256,671A CA256671A CA1061617A CA 1061617 A CA1061617 A CA 1061617A CA 256671 A CA256671 A CA 256671A CA 1061617 A CA1061617 A CA 1061617A
- Authority
- CA
- Canada
- Prior art keywords
- core
- preform
- deposited
- forming materials
- end regions
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
- C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
- C03B37/01—Manufacture of glass fibres or filaments
- C03B37/012—Manufacture of preforms for drawing fibres or filaments
- C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
- C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/262—Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Geochemistry & Mineralogy (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Chemical & Material Sciences (AREA)
- Manufacturing & Machinery (AREA)
- Materials Engineering (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Manufacture, Treatment Of Glass Fibers (AREA)
Abstract
A LIMITED MODE OPTICAL FIBER
Abstract of the Disclosure A single mode optical fiber having a central core region of uniform composition and thickness and core end regions each having a core with a graded composition and increasing thickness towards the end of the fiber. The fiber maintains a constant normalized frequency with a minimum bending loss. Methods are also provided for forming the fibers from a fiber optic preform.
Abstract of the Disclosure A single mode optical fiber having a central core region of uniform composition and thickness and core end regions each having a core with a graded composition and increasing thickness towards the end of the fiber. The fiber maintains a constant normalized frequency with a minimum bending loss. Methods are also provided for forming the fibers from a fiber optic preform.
Description
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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.
- 2a -~ .
:
. ' ' , 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-~
i, . .
.
,, ,~
: ' , .
~6 1L6~7 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, , ~
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 .. . ..
,. ~
,. : , :
M.M.Ramsay~P.W.Black 16-9 ~ 6 ~L617 ( Revi s ion ) 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.
~,. . . .. ... . .. ... .
' . . ` ~' , -6~7 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 ~6~G~ ~
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 ~, . . . ................... .. . . . . .............. .. .
: . . .. . . . . ..
6~61~ M.M. Rarnsay-P.W. Black 16-9 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.
:,.
_ ~ -8-.
', . .. - : ' . : .. , : . : ., , , : , . ~ , . , ,,, : .~
06~
- ~ M.M. Ramsay-P.W. Black 16-9 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 -8a-~, .
~ ... .. . ,. ' : :
6~
M.M. Ramsay-P.W. Black 16-9 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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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 .`''.
~". :.
C ~ - 2 -.
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.
- 2a -~ .
:
. ' ' , 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-~
i, . .
.
,, ,~
: ' , .
~6 1L6~7 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, , ~
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 .. . ..
,. ~
,. : , :
M.M.Ramsay~P.W.Black 16-9 ~ 6 ~L617 ( Revi s ion ) 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.
~,. . . .. ... . .. ... .
' . . ` ~' , -6~7 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 ~6~G~ ~
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 ~, . . . ................... .. . . . . .............. .. .
: . . .. . . . . ..
6~61~ M.M. Rarnsay-P.W. Black 16-9 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.
:,.
_ ~ -8-.
', . .. - : ' . : .. , : . : ., , , : , . ~ , . , ,,, : .~
06~
- ~ M.M. Ramsay-P.W. Black 16-9 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 -8a-~, .
~ ... .. . ,. ' : :
6~
M.M. Ramsay-P.W. Black 16-9 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.
, -9- ,'~:
,~ .
: ,: . .. .. . :, , :, . . ... :
Claims (19)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. 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 improve-ment 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 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.
depositing core forming materials along the length of the preform;
maintaining a substantially constant thickness and composition of the deposited core 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.
2. A method as described in claim 1, wherein the step of depositing core forming materials includes the steps of providing a flow of vapor reagents and thermally inducing a chemical vapor reaction.
3. A method as described in claim 2, wherein the step of increasing the thickness of the deposited core forming materials is achieved by varying the rates of flow of vapor reagents while the reaction is confined to a localized zone traversed along the preform.
4. A method as described in claim 2, wherein the step of increasing the thickness of the deposited core forming materials is achieved by confining the reaction to a localized zone of the preform and controllably moving the localized zone along the preform.
5. A method as described in claim 1, wherein the composition of the deposited core forming materials in the graded core end regions is changed such that the refractive index of the deposited core forming materials decreases along the lengths of the graded core end regions as the ends of the preform are approached.
6. A method of fabricating a limited mode optical fiber, comprising the steps of:
forming an optical fiber preform by providing a cladding layer and a core disposed within said cladding layer, the core being provided by controllably depositing core-forming materials along the axial length of the preform, said step of controllably depositing including maintaining a substantially constant thickness and composition of the deposited core 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 ap-proached 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 material along the lengths of the graded core end regions so that the preform will produce a fiber having a substantially constant normalized frequency throughout the length of the optical fiber;
and drawing the preform into an optical fiber having a central core region of substantially constant core thickness and composition and graded core end regions having increasing thicknesses towards the ends of the preform and changing composition in accordance with the increased thickness such that the central core region and the graded core end regions have substantially the same normalized frequency.
forming an optical fiber preform by providing a cladding layer and a core disposed within said cladding layer, the core being provided by controllably depositing core-forming materials along the axial length of the preform, said step of controllably depositing including maintaining a substantially constant thickness and composition of the deposited core 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 ap-proached 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 material along the lengths of the graded core end regions so that the preform will produce a fiber having a substantially constant normalized frequency throughout the length of the optical fiber;
and drawing the preform into an optical fiber having a central core region of substantially constant core thickness and composition and graded core end regions having increasing thicknesses towards the ends of the preform and changing composition in accordance with the increased thickness such that the central core region and the graded core end regions have substantially the same normalized frequency.
7. The method of claim 6 wherein the normalized frequency of the fiber is held within limits which provide the central core region and the graded core end regions with the same set of bound non-radiating frequency modes.
8. The method of claim 7 wherein the central core region and the graded core end regions are provided with the same bound non-radiating frequency mode.
9. The method of claim 6 wherein the core-forming material is deposited on the bore of a lower refractive index tube that forms the cladding layer.
10. The method of claim 9 wherein the lower refractive index tube is formed by depositing cladding materials by a thermally in-duced chemical vapor reaction.
11. The method of claim 6 wherein the core material is doped silica and the cladding material is undoped silica.
12. The method of claim 6 wherein the core material and the cladding material are both made of doped silica.
13. The method of claim 6 wherein the step of depositing core-forming materials includes the step of thermally inducing a chemical vapor reaction.
14. The method of claim 13 wherein the step of thermally in-ducing a chemical vapor reaction includes inducing an oxidation re-action from which hydrogen and hydrogen-containing compounds are excluded.
15. The method of claim 13 wherein the step of increasing the thickness of the core forming material is achieved by varying the rates of flow of the vapor reagents while the reaction is con-fined to a localized zone traversed along the tube.
16. A method as claimed in claim 15 wherein the localized zone is traversed along the tube at a constant rate.
17. The method of claim 13 wherein the tube is rotated about its axis during the chemical vapor reaction.
18. 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.
19. The optical fiber of claim 18 wherein the normalized fre-quency is substantially constant over the length of the two graded end regions and the intervening central region.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB3009575A GB1469073A (en) | 1975-07-17 | 1975-07-17 | Optical fibre |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1061617A true CA1061617A (en) | 1979-09-04 |
Family
ID=10302205
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA256,671A Expired CA1061617A (en) | 1975-07-17 | 1976-07-09 | Limited mode optical fiber |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JPS5212846A (en) |
| CA (1) | CA1061617A (en) |
| CH (1) | CH600356A5 (en) |
| FR (1) | FR2318120A1 (en) |
| GB (1) | GB1469073A (en) |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3823995A (en) * | 1972-03-30 | 1974-07-16 | Corning Glass Works | Method of forming light focusing fiber waveguide |
| JPS5522423A (en) * | 1978-08-03 | 1980-02-18 | Nippon Baruji Kogyo Kk | Operating method of lateral push cylinder in hydraulic bulge forming device |
-
1975
- 1975-07-17 GB GB3009575A patent/GB1469073A/en not_active Expired
-
1976
- 1976-07-09 CA CA256,671A patent/CA1061617A/en not_active Expired
- 1976-07-15 CH CH906376A patent/CH600356A5/xx not_active IP Right Cessation
- 1976-07-16 JP JP8407776A patent/JPS5212846A/en active Pending
- 1976-07-16 FR FR7621822A patent/FR2318120A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| GB1469073A (en) | 1977-03-30 |
| JPS5212846A (en) | 1977-01-31 |
| FR2318120A1 (en) | 1977-02-11 |
| FR2318120B1 (en) | 1980-04-18 |
| CH600356A5 (en) | 1978-06-15 |
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