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
  • C03B37/01807—Reactant delivery systems, e.g. reactant deposition burners
  • C03B37/01838—Reactant delivery systems, e.g. reactant deposition burners for delivering and depositing additional reactants as liquids or solutions, e.g. for solution doping of the deposited glass
• • 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
  • C03B37/01807—Reactant delivery systems, e.g. reactant deposition burners
• • 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
  • C03B37/02—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor
  • C03B37/025—Manufacture of glass fibres or filaments by drawing or extruding, e.g. direct drawing of molten glass from nozzles; Cooling fins therefor from reheated softened tubes, rods, fibres or filaments, e.g. drawing fibres from preforms
  • C03B37/027—Fibres composed of different sorts of glass, e.g. glass optical fibres
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C25/00—Surface treatment of fibres or filaments made from glass, minerals or slags
  • C03C25/10—Coating
  • C03C25/104—Coating to obtain optical fibres
  • C03C25/105—Organic claddings
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2201/00—Type of glass produced
  • C03B2201/06—Doped silica-based glasses
  • C03B2201/30—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi
  • C03B2201/34—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi doped with rare earth metals, i.e. with Sc, Y or lanthanides, e.g. for laser-amplifiers
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2201/00—Type of glass produced
  • C03B2201/06—Doped silica-based glasses
  • C03B2201/30—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi
  • C03B2201/34—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi doped with rare earth metals, i.e. with Sc, Y or lanthanides, e.g. for laser-amplifiers
  • C03B2201/36—Doped silica-based glasses doped with metals, e.g. Ga, Sn, Sb, Pb or Bi doped with rare earth metals, i.e. with Sc, Y or lanthanides, e.g. for laser-amplifiers doped with rare earth metals and aluminium, e.g. Er-Al co-doped
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B2207/00—Glass deposition burners
  • C03B2207/80—Feeding the burner or the burner-heated deposition site
  • C03B2207/90—Feeding the burner or the burner-heated deposition site with vapour generated from solid glass precursors, i.e. by sublimation

Definitions

• This invention relates to a method for the manufacture of an optical fiber preform having incorporated therein a predetermined and enhanced amount of rare earth dopant material, and particularly, wherein the rare earth dopant material is incorporated at a comparatively high concentration and with a cross- sectional geometry of the preform designed to promote good mode scrambling.
• Optical fibers are essentially ultra thin light conduits. Light is pumped into one end, propagates forward within and through the fiber, whether bent or straight, and ultimately emerges at the other end. By pumping light into the fiber in a predefined pattern, huge amounts of information can be communicated over large bandwidths over long geographic distances almost instantaneously (i.e., at the speed of light). Thin, fast, and robust, the utility of optical fibers is beyond question.
• the central underlying structure found in virtually all optical fibers is a light transmitting core surrounded by a cladding layer.
• the indices of refraction of the core and the cladding are adjusted during manufacture to provide the cladding with an index of refraction that is less than that of the core.
• an optical fiber is typically drawn from an optical fiber preform that essentially has the same cross-sectional geometrical arrangement of core and cladding components as that of the final optical fiber, but a diameter several orders of magnitude greater than that of the fiber.
• One end of the preform is heated in a furnace to a soft pliable plastic consistency, then drawn lengthwise into a fiber having the desired fiber core/cladding dimension.
• Patent '816 relates to the establishment of a more prominate homogeneous reaction where the reaction product from the vapor phase forms glass precursor particulates within the gas stream within the ambient of the refractory tube which particulates are then subsequently deposited downstream of the heat zone or source on the inner surface of the tube. The deposited particulates are then consolidated into a transparent glass layer on the tube surface by the passing heat zone.
• Active optical fibers are employed as fiber gain media for purpose of signal amplification of fiber laser applications and are comprised of a single mode fiber or a double clad fiber with a core composition doped with 4f rare earth elements (i.e., the lanthanide series of element, atomic numbers 57-71), e.g. erbium or ytterbium or co-doped with erbium and ytterbium.
• rare earth elements i.e., the lanthanide series of element, atomic numbers 57-71
• erbium or ytterbium or co-doped with erbium and ytterbium e.g. erbium or ytterbium or co-doped with erbium and ytterbium.
• An appropriately tuned core, surrounded with an appropriate cladding configuration can provide, in combination with an appropriate pump source, the basis for light lasing and/or light amplifying functionality.
• optical fibers capable of such light intensifying functionality are desirable.
• rare earth doping is not easy performed, particularly at high levels of concentrations in the core.
• fiberoptic lasers even if a suitable fiber optic preform is made, the lasing efficiency of a fiber drawn therefrom may still suffer in other respects.
• the performance of fiber lasers, as in any active or nonlinear waveguide, is related intimately to the efficiency with which pump radiation can be absorbed by the active material in the fiber core. In the earliest fiber lasers, an appreciable amount of the radiant energy pumped into the fiber would not pass into the core, and, thus, did not contribute to the core's lasing effect.
• a methodology for the manufacture of an optical fiber preform having incorporated therein a comparatively high concentration of rare earth dopant material, and which thus can be drawn and processed into an optical fiber having low numerical aperture, low core attenuation, and high pumping power absorption.
• the high concentrations of rare earth dopant material are attained in the practice through the employment of either what we refer to as the "hybrid vapor processing” (HVP) method or a “hybrid liquid processing” (HLP) method, each capable of being practiced in combination or independently of one another.
• HVP hybrid vapor processing
• HLP hybrid liquid processing
• the HVP method involves the vaporization of a solid state form of a rare earth chloride by the exposure thereof to a sufficiently elevated temperature, contemporaneously with the transport of the resultant rare earth chloride laden vapor into an oxidation reaction zone within the bore of a hollow refractory tube on a flowing stream of essentially unreactive inert gas, such as helium.
• a vapor of glass forming material e.g., SiCl 2
• SiCl 2 e.g., SiCl 2
• the soot layer is deposited on the inner surface of the bore of the refractory tube by oxidation of constituents comprising the rare earth chloride laden vapor and the vapor of glass forming material.
• the hollow tube is thereafter collapsed to form the optical fiber preform.
• the term, "soot layer”, is a deposited layer having a large amount of porosity and is not fully sintered to form a glass or amorphous layer and, therefore, lacking any optical transparency, optical properties and homogeneity as found in monolithic glass layer formed after a high temperature sintering step.
• An important feature of the HVP method is the employment of a rare earth dopant deliver system that provides for rare earth laden vapor from its solid state form in advance of mixing with oxygen or oxides of glass forming materials introduced in the vapor phase deposition (VPD) process.
• VPD vapor phase deposition
• the rare earth vapor comes in contact almost immediately with oxygen or oxides thereof. We have found that this has a definite and profound effect on the uniformity of constituents in the deposited soot layer deposited on the inside of the refractory tube. There is not a uniform incorporation of the rare earth component on a continuous, repeatable basis let alone the incorporation of intermediates that function as homogenizers.
• the HVP method of this invention provides for uniform, repeatable incorporation of rare earth and/or intermediate components with comparatively high levels of concentration through the employment of the novel delivery system of this invention forming a layer of high optical homogeneity.
• high optical homogeneity we mean that the resultant layer of deposited and sintered monolithic glass that has irregularities in the deposited glass material less than about 2 ⁇ m in width or less. Anything larger than this is referred as having heterogeneity and considered unacceptable in that the glass material has not been fully reacted and undergone a sufficient transformation into an amorphous, monolithic glass layer with uniform mixed glass components including intermediates and rare earth dopant uniformity.
• the HLP method involves the method of depositing a glass layer or layers containing a first amount of rare earth dopant material on the inner surface of a refractory tube forming a soot layer or layers on the internal surface.
• the layer or layers are deposited at a temperature to provide a soot consistency with multiple pores without transformation into a continuous monolithic glass layer.
• This step may be carried out employing the HVP method or employing a standard VPD process of the prior art.
• the soot-deposited refractory tube is then removed the preform lath and impregnated with a dopant solution formulated with a second amount of rare earth dopant material.
• the tube is then return to the preform lath, heated to sinter the doped impregnated layer or layers and thereafter collapsed, resulting in an optical fiber preform with a final amount of rare earth dopant material that includes substantially both the first and second amounts of rare earth dopant material.
• An optical fiber preform made according to the HVP or the HLP method can be employed with its geometry as formed, or the preform geometry may be modified before the preform is drawn into fiber to introduce deviations in the optical properties of the glass preform, e.g., a light scattering mechanism.
• Mechanical grinding or a chemical process may be employed to form a simple flat or concave surface on at least along one longitudinal surface of the are quite suitable for changing the preform geometry prior to the fiber drawing process and the formation of an outer cladding layer as in the case of drawing a double clad preform or a sleeve as may be the case in drawing a single mode fiber. More than one flat can be applied to the preform such as on opposed longitudinal surfaces of the glass preform.
• it is principal object of this invention is to provide improved methodology and apparatus for the manufacture of an optical fiber preform having a comparatively high rare earth dopant concentration, particularly, wherein the limited total doping concentration of the glass fiber preform is at a level sufficient to effect a low numerical aperture in an optical fiber prepared therefrom.
• halogen-based dopant materials e.g., aluminum chlorides, rare earth chlorides, etc.
• CP 3 rare earth cyclopentadienide
• a rare earth halogen i.e., a rare earth chloride
• Fig. 1 is a schematic illustration of a vapor phase deposition (VPD) apparatus for depositing layer or layers of soot or amorphous, monolithic glass on the inner surface of a hollow support refractory tube in the practice of the methods according to this invention.
• VPD vapor phase deposition
• Fig. 2 is a cross-sectional schematic illustration of an optical fiber preform made in accordance with an embodiment of this invention, and is subsequently collapsed into a glass preform.
• Fig. 3A is a schematic illustration of the drawing of a glass preform into an optical fiber.
• Fig. 3B is a schematic illustration of an apparatus for coating additional cladding material and/or polymeric protective layer(s) onto the drawn optical fiber of Fig. 3A.
• Fig. 4 is a graph illustration of attenuation data versus wavelength for an optical fiber made in accordance with an embodiment of this invention.
• Fig. 5A is a cross-sectional schematic illustration of a double clad optical fiber made in accordance with an embodiment of this invention.
• Fig. 5B is a cross-sectional schematic illustration of another double clad optical fiber made in accordance with another embodiment of this invention.
• Fig. 6 is perspective view of a rare earth chloride vessel or boat useful in the practice of an embodiment of this invention.
• Fig. 7 is a first modified version of the glass tube delivery system of Fig. 1.
• Fig. 8 is a second modified version of the glass tube delivery system of Fig. 1.
• Applicability of the present invention is directed to the manufacture of performs drawn to form double clad fibers but equally suitable for manufacture of preforms for other type of fibers including single mode fibers or multimode fibers.
• the embodiments of the invention have particular adaptability to single and double clad fiber utilized as active gain media, e.g., as fiber amplifiers or fiber lasers.
• active gain media e.g., as fiber amplifiers or fiber lasers.
• the principal of the methods employed herein are also applicable to enhancement of other constituents or components in fiber glass preforms, other than just rare earth enhancement, where such a constituent or component is enhanced by the methodology of either or both the HVP or HLP method.
• An example is the enhancement of phosphatic incorporation in the glass forming layer deposited on the inner tube surface, beside or in lieu of a rare earth component.
• a double clad fiber useful for high power fiber amplifier or laser applications, comprises a core 14, an inner cladding 5, an outer cladding 40, and an optional protective outer jacket 50.
• the double clad fiber design may include an additional inner cladding layer 12 having a comparatively thin cross section forming an interface between core 14 and cladding 5.
• the portion of the fiber that this drawn from a glass preform, such as preform 10 " in Fig. 2, is designated at 10 in Figs. 5A and 5B.
• Inner cladding 5 functions as a waveguide by means of internal reflection of the radiation occurring at the interface lying between inner cladding 5, with an index of refraction n 2 , and outer cladding 40, with a lower index of refraction n 3 .
• the purpose of inner cladding is to confine radiation launched into the inner cladding so that it repeatedly intersects the core 14 as it propagates along the length of the fiber.
• an active gain dopant e.g., rare earth dopant
• the length of an optical fiber is typically tens, or possibly hundreds, of meters allowing for a large number of these core interaction permitting as much as possible the absorption of the pump radiation with the core.
• the embodiments of the present invention focus on the design and manufacture of the optical fiber preform 10" from which the central optical strand 10 of the fiber is drawn, i.e., a strand comprising the fiber core and inner cladding structure.
• the structural and compositional configuration of the rod-like preform 10 though reduced greatly in cross- sectional diameter, can be translated accurately into the much longer filamentary fiber 10 as drawn from preform 10 " .
• rare earth dopant material can be incorporated into an optical fiber preform either by a method involving the delivery of rare earth halogens to a glass forming reaction zone under high vapor pressures and substantially free of oxides and moisture content (i.e., the hybrid vapor processing or HVP method), or by a method involving an innovative combination of soot deposition and solution doping processing (i.e., the hybrid liquid processing or HLP method). Either of these methods are capable of being used alone or in combination with one another.
• an oxidizable rare earth halogen such as a rare earth chloride vapor having desirably higher vapor pressure is directly produced by vaporizing, in the presence of high temperature in an environment free and regulated from oxygen and moisture free, a solid form of the rare earth halogen.
• This streamlined process provides advantages with respect to efficiency, uniformity, and concentration yield that cannot be easily duplicated when oxidizable rare earth, for example, vapors are generated in the multi-step processes common among the prior art such as disclosed, e.g., in U.S. Patent No. 4,616,901, to MacChesney et al.
• the vapor phase deposition (VPD) apparatus of this invention employs a novel designed delivery system 20 comprising a plurality of multi-concentric quartz glass tubes 200, 220, and 240, with a forward flow/anti- back flow regulator 202 and 222, e.g., a permeable quartz glass fit interface or other such inert permeable material, sealing the output end of each of the inner concentric delivery tubes 200 and 220.
• a forward flow/anti- back flow regulator 202 and 222 e.g., a permeable quartz glass fit interface or other such inert permeable material, sealing the output end of each of the inner concentric delivery tubes 200 and 220.
• the multi-concentric delivery system 20 of the HVP apparatus comprising this invention allows for the regulated delivery of various vapor laden gaseous material into the bore of quart tube 5 without back flow contamination by oxygen being provided via outer concentric delivery tube 240.
• the uncontaminated vapors of refractive index and rare earth dopants provided from delivery tubes 200 and 220 are first reacted under the influence of burner 340 in a definable reaction zone 5B along the length of tube 5 with other gases and vapor laden glass forming components via outer delivery tube 240 for forming on inner surface 5A one or more layers of monolithic glass or other particulate or soot layers.
• the VPD apparatus is flexible in allowing, with minor modification, for the carrying out of various vapor phase deposition procedures.
• assembly 42 is shut-off, isolated, removed, or otherwise taken "off-line", and a solid form of a rare earth halogen, such as a rare earth chloride, is loaded into a boat 32 positioned inside central delivery tube 200.
• vapor rare earth supply source 42 may activated or placed "on-line” in combination with the vaporization of rare earth chloride from boat 32.
• either of these rare earth chloride supply sources can be employed without the other.
• a fused silica quartz tubular chamber 24 contains a rare earth power or liquid constituent (NdCl g ) heated to about 1,000°C by heater 18 in essentially an oxygen free ambient due to the use of silica wool 25 placed at the output end of chamber 24 which prevents the entrance of contaminants into chamber 24.
• NdCl g rare earth power or liquid constituent
• the difficulty with this approach is that as the rare earth material is spent in carrying out the process, the surface area of the chunk, pile or irregular form of the rare earth would change over time, and, as a result, the concentration of rare earth dopant introduced into the process correspondingly changes over time.
• the same is is true relative to a liquid pool of rare earth with unmaintained boundaries.
• the total quantity of rare earth in its vapor form is a function of temperature, vapor pressure at a given temperature and surface area of the exposed form of the rare earth constituent.
• a melt of rare earth powder is prepared within cavity 34 to form a solid integral form of the rare earth chloride with the dimensions 38 and 40 of boat 32.
• This preparation is carried out in an inert ambient, without oxygen, such as helium, at around 900°C, for example.
• the boat is placed in a hermetically sealed chamber containing a powder form the rare earth, e.g., Yb.
• the chamber is provided with a halide gas, e.g., Cl 2 , and an inert carrier gas, e.g., He.
• the temperature of the chamber is elevated to around 500°C or so to drive off an carry away via a chamber exhaust system, water vapor and oxygen according to following formula: Yb(OH ⁇ YbCl + 6H 2 0
• This form of the rare earth is high desirable since it provides for low vapor pressure that 5 does not include oxygen.
• the inclusion of or presence of oxygen in rare earth will increase its vapor pressure.
• the temperature of the boat is raised further to about 900°C to melt the rare earth power and provide an exposed surface having a two dimension of 38 and 40 as shown in Fig. 6.
• the prepared rare earth boat is then permitted to cool and transferred directly for placement within delivery tube 200 to prevent any prolonged exposure to an oxidizing atmosphere.
• Boat 32 is inserted in the downstream end of tube by removal of flow/anti-back flow interface 202 and its replacement with the same interface, or a new regulator if the older one is significantly obstructed by significant deposits of oxides on its outer surface, after insertion of boat 32 within the tube end.
• an inert gas flow is initiated with heater 360 operated at a low temperature (500°C) to remove any moisture and oxygen absorbed during the transfer to delivery system 20. Then, for generation and delivery of a vapor form of the rare earth from boat 32, heating source is taken up to a temperature of about 1,000°C or so to provide a high vapor form of the rare earth carried by the inert gas, helium.
• a low vapor pressure solid source for the rare earth constituent is provided without any contamination with moisture or oxygen.
• a previously used boat can be recharged with rare earth material by adding additional rare earth material to the remaining rare material in the boat, dehydrating and deoxidizing the recharged boat and thereafter melting the rare earth material to integrate the material as a single mass.
• an inert carrier gas such as helium
• mass flow controllers are electronic flow regulating gas sources well known in the art.
• Columns 122 and 124 serve as sources of various kinds of vapor laden gases.
• column 122 is a source of an intermediate (homogenizer) such as a A1C1 3 vapor, which is an refractive index modifying material.
• Other intermediates or combinations of intermediates may be employed such as Ga, In, As and/or Sb halide vapors. These intermediates provide for homogeneity in the mixing of constituents in the glass forming process.
• Column 124 is a rare earth containing rare earth chelate vapor source.
• one particularly useful rare earth containing vapor is a "rare earth" - cyclopentadienyl (RE-CP 3 ) vapor, e.g., Yb(C 5 H 5 ) 3 or Er(C 5 H 5 ) 3 or a combination of such rare earth vapors.
• the chemical formula of the "rare earth” - cyclopentadienyl compound is RCP 3 where CP 3 may be the hydrocarbon (C 5 H 5 ) 3 and R is a rare earth element, such as, but not limited to, neodymium (Nd), ytterbium (Yb), erbium (Er), thulium (Tm), holmium (Ho), and samarium (Sm).
• Neodymium cyclopentadienyl vapor is favorable as a rare earth chelate vapor source 124 in that can be oxidized at temperatures greater than 1,000°C to form a useful active gain, Nd-cyclopentadienide dopant.
• Columns 122 and 124 preferably are heated to a maximum temperature in the area of about 220°C. Of course, different temperatures may be needed for different rare earth material due to differences in vapor pressure for each of the different rare earth compounds.
• the vapor phase of these source materials from of columns 122 and 124 are provide to delivery system 20 respectively via three way stopcocks 132 and 134.
• Additional inert carrier gas e.g. helium, may be supplied from controller 110 via line 144 and stopcock 130 to regulate the amount of inert gas to provided to delivery tube 200.
• Stopcocks 132 and 134 have vents for venting the intermediate (A1C1 3 ) and rare earth chelate vapors, respectively, as columns 122 and 124 are brought to a desired equilibrium gas flow and temperature condition prior to diversion of the vapor stream into the transport line leading to the CVD reactor comprising rotated tube 5 and its heating zone. Stopcocks 132 and 134, being independently controllable, are used to divert a desired mix of vapor streams into the reactor system when deposition of the rare earth containing soot begins, as well as shut off the vapor supply at the end of the required deposition.
• transport lines 142 and 144 comprise a 0.25 inch (0.63 cm) diameter Teflon ® lines surrounded by a 1.5 inch (3.8 cm) copper tube.
• the temperature of transport lines 142 and 144 is maintained at a temperature which is sufficiently high that the vapor materials being transported do not condense therein.
• a thermocouple temperature sensor to control the temperature in transport lines 142 and 144 to within about 2°C to 3°C. This should provide sufficient accuracy to control the temperature and vapor pressure of the vapor materials transported therein to ensure stability.
• Transport lines 142 and 144 are respectively coupled to separate concentric delivery tubes 220 and 200, of delivery and reactor system 20.
• a mixture of unheated gases from sources enter outer concentric delivery tube 240.
• the mixture of gases comprise He, 0 2 , Cl 2 , SiCl 4 , GeCl 4 , POCl 3 , BBr 3 , SF 6 , and CF 4 .
• Other gases may possibly be Si0 2 , P 2 0 5 , A10 3 , MgO, CaO or K ⁇ O.
• active fibers will employ either Al or P as index altering components, but such elements are not always employed in the manufacture of transmission fiber.
• the down stream end of inner delivery tube includes a support 25 vessel or boat 32 for holding a solid form of an element or compound to vaporized in a gaseous state by means of the application of high temperature in a high temperature zone 362 that includes boat 32 within tube 200.
• the heat zone 362 is created by another heating element 360 which is axially disposed about the concentric tubes 200, 220, 240 as well as refractory tube 5 of system 20.
• Any type of well known heating element can be employed, e.g., a rf coil arranged about the concentric tubes, may be employed as a heater and the boat 32 of a material high absorptive of the rf radiation.
• An example employed is a coaxially circumscribing heating element 360 comprising a nichrome wire wrapped on a small bore alumina tube, which was found adequate for performing the task of vaporizing a rare earth element in boat 32. Due to the concentricity of their arrangement, the temperature in the innermost delivery tube 200, being closest to the thermal focus of heating element 360, is the highest, and the temperature in outermost chamber 240, being furthest from the thermal focus of heater 360, is the lowest among the several delivery tubes 200, 220, and 240.
• the rare earth con-taming vapors when introduced from source 42 and are transported through central delivery tube 200, these vapors will encounter the high temperatures at zone 362, e.g., in the range of about 800°C to about 1,200°C, to effect the high vapor pressures appropriate for achieving a uniform incorporation of rare earth elements on surface 5A.
• the temperature employed via annular heater 360 is dependent upon the rare earth dopant concentrations desired in the glass layer to be deposited since the vapor pressure developed at rare earth chloride boat 32 is dependent on the applied heater temperature.
• Delivery system 20 is coaxially inserted into and arranged in relation to hollow tube 5, for example, through a rotatable, gas- impermeable seal, such that hollow tube 5 can be rotated at a desired rpm about its axis during the process of depositing a glass layer on tube surface 5A.
• the rate of rotation employed in conjunction with the methods disclosed herein are in the range of about 40 rpm to about 60 rpm, or generally around 50 rpm. Rotation can be effected by employing a lathe or the like as is well known in the art. In the high speed production of preforms taught in patent 4,909,816, and its companion patent 4,217,027, the lath rpm is around 100 rpm.
• reaction zone 5A established by elevated temperatures effected by ribbon gas burner 340 which may be moved transversely and reciprocally along the length of tube 5, as indicated by arrow d, exclusive, of course, of the region of tube 5 housing delivery system 20.
• ribbon gas burner 340 which may be moved transversely and reciprocally along the length of tube 5, as indicated by arrow d, exclusive, of course, of the region of tube 5 housing delivery system 20.
• Ribbon burner 340 which is axially transportable in upstream and downstream directions along the transverse directions, d, generally comprises a quartz or metal tube having a slit. H, is injected therein and ignited to provide a flame which heats hollow tube 5 to be heated to the desired temperatures needed to effect soot formation and deposition on surface 5A.
• a soot layer is generally deposited in an area a few inches downstream from the reaction zone SB, as is known in the art.
• ribbon burner 340 is transported down the length of tube 5 in downstream direction. Layers of soot or amorphous glass on surface 5A, depending on the reaction temperature providing by burner, are created by sequential passes of the burner 340 down the length of tube 5 in the downstream direction.
• the quartz tube 5 is collapsed to form the finished optical fiber preform. Collapsing is occasioned by exposure of quartz tube 5 to extremely high temperatures, i.e., temperatures high than glass depositing and sintering temperatures, as is known in the art. Good results in collapsing are achieved when quartz tube 5 is rotated along its axis in conjunction with ribbon burner 340 traversing the length of tube 5, i.e., in the upstream direction.
• the employment of the VPD apparatus for conducting the HVP method includes modification to the generally employed VPD apparatus for the MCVD method, in particular, employment of rare earth boat 32, provision of regulators 202 and 222 to prevent the pre-contamination including the pre-oxidation of vapor laden dopants to be delivered to the reaction zone SB, and the generation of a helium streams through components 110 and 116 as an inert carrier gas for the vapor laden dopants.
• regulators 202 and 222 provision of regulators 202 and 222 to prevent the pre-contamination including the pre-oxidation of vapor laden dopants to be delivered to the reaction zone SB, and the generation of a helium streams through components 110 and 116 as an inert carrier gas for the vapor laden dopants.
• quartz glass frits interfaces 202 and 222 Quartz- glass frits 202 and 220 are permeable causing the rare earth chloride vapor laden carrier gas to be dispersed into fine, relatively homogenous gas streams through the frits 202 and 222.
• gas streams in delivery tubes 200 and 220 initially mix, at the downstream end of delivery tube 220 after passage of the rare earth gas stream through frit interface 202, with intermediates (homogenizers), such as Al, Ga, In, As and/or Sb halide vapors.
• halide vapor of the glass forming constituents such as the halides of silicon, germanium, born and/or phosphor.
• Use of the fits interfaces 202 and 220 provides for soot or monolithic glass depositions having enhanced uniformity of deposited components making of the layer or layers as compared to the conventional MCVD method, particularly with the incorporation of rare earth dopants into the deposited layer.
• frits 202 and 222 function to prevent premature particulate formation, thus allowing homogenous vapor to issue from the internal delivery system prior to exposure to elevated temperatures effected by heat source 340. This promotes the homogeneity of the mixture of vapor with dopant vapor, leading to uniformly doped particles. Further, it is also believed that fits 202 and 222, in combination with the downstream flow of gas, reduces considerably the back flow of oxygen into the area of delivery tube 200 wherein rare earth chloride vapors are being directly produced in the HVP method of this invention.
• the back flow of oxygen can result in the particle-forming oxidation of the rare earth chloride vapor prematurely. If particles are formed prematurely, as they enter the reaction zone at SB, they will grow by accretion, and will be ultimately deposited downstream, resulting, when the deposited layer becomes sintered, in a portion of the formed glass having characteristics different from surrounding areas, a defect known in the art as a "bubble".
• the presence of "bubble” significantly reduces the optical efficiency of a drawn optical fiber and, generally, if the bubble is an entrapped air bubble as large as the soot particles, one such bubble can render the resultant glass preform useless for fiber drawing.
• the HVP method of this invention provides for higher uniformity in the presentation of glass forming materials with the use of the delivery system 20 comprising this invention, particularly relative to higher and maintained concentrations of incorporated rare earth dopants resulting in higher homogeneity in the resultant glass formed layer or layers within the glass or refractory tube than is possible with the MCVD method.
• the HVP method is initiated with the provision of a solid form of a rare earth chloride in vessel or boat 32 in, for example, the VPD apparatus illustrated in Fig. 1.
• the solid rare earth chloride source 32 is exposed, under essentially oxygen- and moisture-free condition, to a temperature elevated sufficiently high to vaporize at least a portion of the solid rare earth chloride under appropriate vapor pressure forming, in a continuous manner, a rare earth chloride laden vapor.
• the rare earth chloride laden vapor is carried on a flowing stream of inert gas of helium to reaction zone 5A within the bore of a hollow tube 5, contemporaneously with a vapor of material capable of forming glass when exposed to elevated temperatures.
• the temperature of reaction zone 5A is then elevated to form from the rare earth chloride laden vapor and the vapor of glass forming material at least one layer of soot on surface 5A of tube 5.
• the deposition is usually carried out in upstream passes because the upstream passage provides for a deposited, unsintered soot layer having greater porosity which is useful in the application of the subsequent step of rare earth liquid imbibition of the soot layer.
• deposition in patent 4,909,816 is carried out in the downstream direction to provide for a two-step operation wherein the soot layer is formed in conjunction and simultaneously with the sintering of the same deposited layer employing comparatively higher deposition temperatures immediately after which the tube is collapsed to quickly form a glass preform.
• the first step is deposition of a soot layer
• the second step of sintering the soot layer and a third step of collapsing the tube.
• a lower temperature is employed during the deposition step to achieve a layer having soot consistency, i.e., more porosity which is avoided in the process of patent 4,909,816 to achieve high volume production of preforms for manufacture of transmission type optical fibers.
• the soot layer is formed on the inside surface 5A of the tube through several passes of hot zone 342 followed, second, by the purging of moisture (H 2 0) and 0 2 from the deposited soot layer with a flow through the tube of SF 6 or Cl 2 (driving of moisture in the form of hydrogen chloride) followed, third, by the step of sintering, at a higher temperature, the dehydrated soot layer.
• glass tube 5 can be immediately collapsed employing a higher collapsing temperature.
• a further advantage of this invention is the provision of multiple premixing tubes where the inner most tube permits the initial establishment of a rare earth component from its solid state form with an inert carrier gas absent any mixing with an intermediate, such as A1C1 3 , as is the case of patent 4,666,247, which raise the vapor pressure required to create the rare earth vapor.
• the embodiment of Fig. 1 the rare earth solid state vaporization process is carried out at a lower vapor pressure in the presence of an inert gas and in the absence of any such intermediate, affecting its vapor pressure, and prior to mixing of the created rare earth vapor with the intermediate in the second tube 220 via interface 202.
• the lower vapor pressure established at boat 32 is only governed by the inert gas and the temperature of boat 32.
• a feature of the present invention is the achievement of uniform continuous quantities of higher concentrations of rare earth in a vapor form carried by an inert gas which are achieved without the presence of oxygen and the use of a boat that continually provides over time a surface exposure of the solid form the rare earth to be vaporized that does not change in dimension.
• annular heater surrounding the rare earth boat, a high vapor pressure can be achieved for providing higher concentrations of rare earth in vapor form prior to mixing with an intermediate, such as A1C1 3 , which is separated from the rare earth generation process prior to intermediate mixing due to flow regulator separation. The presence of such an intermediate during the rare earth generation process in the delivery system will prevent higher level of rare earth concentration in the vapor form.
• the regulated separation is accomplished by employing a first, inert, quartz frit interface that has a predetermined porosity.
• the solid state rare earth source employing an annular heater provides for a high vapor pressure at the rare earth boat resulting in a vapor laden with higher concentrations of rare earth.
• the second, inert, quartz frit interface having a predetermined porosity functions as second flow regulator that permits mixing of the generated, comparatively higher concentrations of rare earth vapor, at least 2% or more, with the intermediate prior to their introduction into the reaction zone with the other glass forming constituents.
• the second fit interface prevents oxygen from feedback into the preparation region of the rare earth constituent.
• the thickness and porosity of the frit interfaces are important to make determinations as the thickness and porosity of the frit interfaces relative to their respective delivery tube diameters, taking into consideration that there is higher pressure developed on the outer tube 220 compared to the inner tube 200.
• the rare earth laden chloride vapor as well as the aluminum chloride laden vapor should be easily penetrable through the frit interface but not so large as to permit the possibility of backward flow of other oxygen entrained gases into tubes 220 and 200.
• oxides of glass forming materials will form, to some extent, on the outside surface 222A of the outer fit interface 222 and not allow oxygen to penetrate through this frit interface.
• the frit interfaces 202 and 22 perform three functions.
• Optical fiber preforms made by the HVP method alone have been measured as having, quite desirably, rare earth chloride concentrations as high as 4 wt%. See e.g., Examples 1, 1A to 1C, ID, and IF, and Examples 3, 3A, 3B to 3C, and 3D, infra.
• Example 1 and Example 3 no appreciable difference in the accomplishment of such high concentrations results from depositing the glass layer immediately as consolidated glass under extremely high reaction temperatures such as exemplified in the MCVD method disclosed in U.S. Patent No. 4,909,816, or as opposed to the stepwise deposition and sintering of particulate soot layers as exemplified in U.S. Patent No. 4,217,027.
• HVP method alone is capable of attaining rare earth dopant concentrations sufficient for most fiber laser applications, still higher concentrations can be attained if used in conjunction with the aforementioned HLP method.
• the predefined concentration and structural distribution of rare earth containing dopant material suited to effect the desired optical properties are built up aecretively by sequential introductions of said dopant material using both the techniques of soot deposition and solution doping.
• an optical fiber preform having desirably uniform concentrations of rare earth containing dopant material in excess of that possible by either process alone can be obtained.
• the process of soot deposition is generally accomplished by first introducing a vapor of glass forming precursor material into the bore of a hollow tube 5, then oxidizing said precursor material at a temperature and for a duration sufficient to effect the deposition onto the surface of said bore of at least one porous or particulate layer of soot.
• the vapor is formulated to include a first amount of a rare earth dopant material.
• the dopant material includes a source for rare earth ions, i.e., ions of elements of the lanthanide series of elements (atomic numbers 57-71).
• the temperature should be high enough for the well known vapor phase oxidation reaction to occur but not high enough to sinter the deposited silica.
• the final result should be a porous or particulate layer with a density of about 0.5 g/cc. At higher temperatures, the oxidized particles will sinter almost immediately upon deposition, and consequently, form a glass layer, which by its monolithic quality, will be incapable of being impregnated with subsequent treatments of liquid doping solutions.
• the dopant solution includes a second amount of dopant material. As with the first amount of dopant material, this second amount also includes a source of rare earth ions. Impregnated in targeted soot layers, the rare earth component becomes available for further incorporation into the resultant optical fiber preform, and hence, available for increasing additively the final rare earth content of the preform.
• the first and second dopant incorporations may also include, in addition, refractive index modulating or modifying components in vapor and liquid form, respectively, e.g., halides or oxides of Al, B or P.
• the soot is soaked in the desired solution for a relatively long period of time (for example, several hours at room temperature).
• the impregnation procedure can be accelerated by application of a vacuum and/or heat.
• a container holding the soot-coated tube is pumped to low pressure (or vacuum, or heated, or vacuumed and heated) then, the solution with dopants is introduced into the container in a volume sufficient to completely soak the soot-coated tube.
• the residue solution is poured away.
• the dopant-impregnated soot is then dried at approximately 150°C to 250°C in ambient air or in an inert atmosphere or under vacuum.
• the temperature is increased to within the range of approximately 750°C to 850°C under oxygen or an oxygen-rich atmosphere to oxidize the rare earth dopant precursor (e.g., rare earth chloride).
• the solution impregnation operation can be repeated several times to increase the dopant concentration.
• Chlorine, SOCl 2 , CC1 4 , or SF 6 gas can be introduced into the atmosphere to promote the dehydration or hydration partition process.
• the hollow tube is heated at a temperature and for a duration sufficient to sinter the soot deposited therein and to collapse the tube, said sintering and said collapsing occurring either contemporaneously or sequentially.
• the doped soot containing tube is collapsed into a consolidated rod-like preform at temperatures in excess of 2,000°C.
• the resulting optical fiber preform 10 will have incorporated therein an amount of rare 15 earth dopant material that includes substantially both said first and second amounts of rare earth dopant material. Since additional opportunities for introducing dopant material may be employed, as well as the possibility of loss of some incorporated material, the final concentrations of material, while evincing the incorporants of both steps, will not necessarily be the exact sum product of both, and could be either more or less.
• the HVP method comprises utilizing a solid state form of a rare earth dopant that has exposed dimensional uniformity during its vaporization. This is exemplified in Fig. 1 by the employment of the rare earth chloride boat 32.
• the HVP method comprises utilizing in combination the rare earth chelate vapor source 124 together with the solid state rare earth source 32 to achieve even higher concentrations of the rare earth constituent or combinations of rare earth constituents (note that these two sources may provide different rare earth components such as Er and Yb for codoping) in the glass forming mixture.
• the HVP method of the first approach is employed in combination with the HLP method to enhance the rare earth dopant concentration in the deposited soot layer.
• the HVP method of the second approach is employed in combination with the HLP method to maximize rare earth dopant concentration in the deposited soot layer which may then be subsequently dehydrated and sintered to form a monolithic glass layer of high rare earth concentration and high optical homogeneity.
• Fig. 7 illustrating a modification to the HVP method comprising this invention.
• the description of components in the Fig. 1 embodiment having identical numerical identification for the same components in Fig. 7 is equally applicable to the components in Fig. 7.
• the modification in Fig. 7 relates to the delivery system 20A which comprises a plurality of rare earth boats 32 and 32A positioned in adjacent relationship to one another.
• Each rare earth boat is prepared according to the method previously described where one boat may be comprised of a halogen of one rare earth and the other may be comprised of a halogen of another rare earth, e.g., ErCl 3 and YbCl 3 , respectively. It is within the scope of this invention that boats 32 and 32A have the same rare halogen or that several such boats can be provided, in which case, each preferably has its own annular heater 360 and 360A with thermal focusing of heat to the region of its respective boat for controlling the vaporization of each respective rare earth source.
• heaters 360 and 360A may be independently controlled to provide desired ratios of the rare earth vapors from respective boats 32 and 32A to be fed, via interfaces 202 and 222, into the main glass forming gas stream and intermixed therewith in region 5B.
• Multi-quartz glass tube delivery system 800 comprises a plurality of delivery tubes 802, 804 and 806 for independently providing a series of gases that are generated and consecutively mixed prior to final mixing with glass forming gases via delivery tube 808, as in the case of the system shown in Fig. 1.
• intermediates e.g., A1C1 3
• the inner most tube 802 may be comprised of a boat 818 of a solid form of a halide rare earth, for example a rare earth chloride, with an inert gas, such as helium provide at inlet 803 to tube 802.
• Heater 822 provides a heated zone 826 to provided for vaporization of the rare earth into a gas entrained form which passes through permeable glass fit interface 812 into tube 806 and heated zone 828 which contains another boat 820 which may be comprised of a second rare earth halogen or an intermediate solid state form that is vaporized at a rate determined by heater 824 and the corresponding temperature of heating zone 828.
• the vaporization of the rare earth in region 826 is accomplished by itself prior to mixing with another vaporized component generated in region 828, both component generated in regions 826 and 828 being highly susceptible to contamination by oxygen or possibly other oxides of glass forming materials when undergoing their generation, but protected from them by means of the glass fit interfaces 812, 814 and 816, in particular, interfaces 812 and 814.
• Other premixing prior to introduction with gas forming, oxygen entrained vapors together with the oxygen-free generated components in tubes 802 and 804 may be accomplished within third tube 806 in its chamber 829 prior to passage through glass frit interface 829 into mixing region 830 with glass forming materials.
• Examples of such mixing may be an inert gas entrained rare earth vapor, an additional intermediate, or a glass dopant.
• delivery tubes 802 and 804 may be positioned in adjacent relation within the interior or bore of tube 816 so that independent generation of separate rare earth solid state forms via independent provided inert gas sources can be provided to achieved the highest possible concentrations of vapor forms of the respective rare earths prior to their mixing at region 829 in tube 806.
• boats 818 and 820 may contain the same or different solid state forms of rare earths or one such boat may contain some other solid state form of material employed in the glass forming process, such as a glass dopant or an intermediate.
• the finished optical fiber preform 10 can be used as the starting material from which any of a variety of fiber optic products can be made. However, in view of the high concentration of rare earth dopant enabled by the practice of the methods described herein, the finished optical fiber preform 10 is especially well-suited for the production of double clad fiber lasers.
• the glass core in a fiber laser like other fiber optic products, is that portion that conducts the light from one fiber end to the other. It can be either single mode or multimode, but it has to contain the active rare earth ions for laser. To help retain the light being conducted within the core, an outer cladding layer surrounding the inner cladding and core of the optical fiber is desirable. Prior to drawing and cladding a fiber in this manner, however, post-collapse pre-drawing modifications to the optical fiber preform, such as "resleeving", for example, should be considered.
• An optical fiber having good absorption efficiency was drawn from a fiber preform having two very small flats ground into the opposite sides of its otherwise round cross-sectional configuration. See, for example, Fig. 2, which illustrated potential sections A-A and B-B that may be removed grinding or other such process forming flats 13 and 15.
• the slight imperfection in the modified preform function essentially as a mode scrambler.
• certain components of that light will internally reflect continuously down the fiber path length, angling off internal path surfaces in consistently repeating geometric patterns, possibly reflecting off all internal surfaces, but failing entirely in its transit to propagate through the fiber's central regions.
• this central area is occupied by the fiber core containing the active gain species that serves to concentrate input light in to lasing radiation. To the extent that input light fails to propagate into the central region, it is not absorbed, and hence, the intensity of resultant lasing radiation is diminished.
• the slight imperfection help to prevent this outcome by essentially changing the fiber internal reflectivity, particularly as it relates to an effective reflection angle, in at least one internal surface of what would otherwise be an unchanging internal circular surface, and, thus, disrupting the possibility of continuously repeating geometric internal reflection patterns.
• One flat has the drawback that the resulting fiber is asymmetric and therefore serves as an impediment to easy fusion splicing or connection to other fiber optic assemblies.
• Three or more flats results in more "corners" than two flats, and thus more internal scattering but, in addition, requires additional work and increases fabrication costs.
• flats 13 and 15 of a depth of about 5% to 10% of the diameter of the fiber inner cladding 5 are sufficient to give good mode- scrambling, with the range of about 1% to 25% providing less desirable, but nonetheless acceptable results.
• a depth of less than 1% produces no notable advantages, and the labor involved in the grounding a flat into the preform at depth of greater than 25% is far greater than that necessary to accomplish desirable mode scrambling, and the benefit of compatibility with other fiber assemblies and fibers is lost.
• the mode scrambling enhancement process can be utilized with preforms made by so called modified chemical vapor deposition (MCVD) processes, such as described in U.S. Patent No. 4,909,816 (MacChesney et al.), or those produced by so called outside vapor phase oxidation (OVPO) process or the outside vapor deposition (OVD) process, such as discussed in U.S. Patent No.
• MCVD modified chemical vapor deposition
• OVPO outside vapor phase oxidation
• OTD outside vapor deposition
• an optical fiber may be formed in the usual manner by inserting one end of the preform into a furnace, as schematically illustrated in Fig. 3 A, to heat the preform. After the preform is heated, a bait rod or other implement can be used to draw the material in one or more steps, into an optical fiber which retains the original cross-sectional configuration of the starting preform.
• outer cladding 40 is deposited onto the drawn fiber, a step that can be effected using any of the conventional techniques known to those skilled in the art.
• the drawn optical fiber is coated with a photopolymerizable composition, such as disclosed in U.S. Patent No. 5,534,558, issued to R. A. Minns on July 9, 1996, employing an apparatus such as disclosed in Fig. 3B.
• the apparatus of Fig. 3B comprises an oven 2 containing the glass preform. Beneath oven 16 are disposed two coating cups 4 and 6, each containing the photopolymerizable outer cladding composition.
• An ultraviolet lamp 8 for example, a Fusion Research electrodeless ultraviolet mercury vapor lamp, is disposed below the coating cup 6, and a capstan 7 is disposed below lamp 8.
• the apparatus further includes a wind-up roll 9.
• These cups are provided with downwardly tapering conical bases, with the apex of each cone having a vertical bore through the cup bottom.
• the diameter of the bore id equal to the desired diameter of the optical fiber coated with the photopolymerizable composition, so that the bore functions to exclude or wipe excess photopolymerizable composition from the fiber.
• the fiber, with the uncured photopolymerizable composition thereon, is then traversed past UV lamp 8, where the solution is cured to produce an adherent clear cladding on optical fiber 10.
• coated clad fiber 10 may be passed through additional coating cups and under an additional ultraviolet lamp to apply a durable outer coating 50 to protect the typically soft outer polymer cladding 40 from damage. After the completion of coating 50, the coated fiber 10 is, then, wound on roll 9.
• the length of the glass fiber for such lasers and/or amplifiers should not be excessively long nor too short to handle.
• substantially all of the incident pump radiation should be absorbed in one or two passes through the fiber, whereas if the apparatus, is employed as an amplifier, substantially all of the incident pump radiation should be absorbed in a single pass through the fiber.
• An optical fiber preform was prepared using the base glass deposition components and parameters set forth in the following Table I.
• a glass-forming vapor comprising the listed components was directed into the bore of a quartz tube 5 through outer delivery tube 240 of the multi-concentric delivery system 20 illustrated in Fig. 1, and reacted to form layers of soot.
• the layers of soot that will later comprise the inner cladding of the preform are sintered at the conclusion of deposition, i.e., after the third "pass".
• a stream of helium gas (having a flow rate of approximately 300 cc/min) was passed through aluminum chloride material, which was loaded and heated to 120°C - 150°C in column 122 shown in Fig. 1.
• a stream of helium gas (also having a flow rate of approximately 300 cc/min) was passed through ytterbium chloride material, which was loaded and heated within the range of about 910°C to 930°C (depending on desired concentration) in a rare earth chloride boat 32 positioned inside the central delivery tube 200.
• the layers of soot were subsequently desiccated by exposure to a 50 cc/min stream of chlorine gas for about one to two hours, then collapsed.
• the core composition of the finished optical fiber preform was determined under standard Electron Probe Microanalysis. The data collected in mol% are as follows: 98.4% Si0 2 , 0.65% AL0 3 , 0.6% Ge0 2 , and 0.3% Yb 2 O 3 . Further analysis revealed that the optical fiber preform had low water content. Attenuation was determined to be as low as approximately 4 dB/Km at lasing wavelengths longer than 1 ⁇ m. The attenuation curve is shown in Fig. 4.
• Examples 1 A to 1 C Three optical fiber preforms (i.e., examples 1 A to 1 C) were prepared by the process used 10 in Example 1. However, for example 1 A, the ytterbium chloride material was vaporized at 930°C, for example IB, at 950°C, and for example 1C, at 980°C°. It was observed that the core compositions of each of examples 1A to 1 C remained the same, but that ytterbium oxide concentrations increased from 1A to 1C, i.e., increased with increasing temperatures used for vaporization. The ytterbium oxide concentration in Example 1C was greater than 3 wt%.
• An optical fiber preform was prepared by the process used in Example 1.
• optical fiber preform had a core composition similar to the Example 1 preform, but had a larger core diameter, and thus, better suited for drawing multimode optical fibers.
• An optical fiber preform was prepared by the process used in Example 1.
• optical fiber preform had an Er 2 0 3 concentration of approximately 2.5 wt%. No devitrification by cluster formation was observed.
• a Yb:Er co-doped optical fiber preform was prepared by the process used in Example 1. However, in addition to ytterbium chloride, erbium chloride was loaded into a second chloride boat, and positioned next to the ytterbium chloride inside central delivery tube 200. Both rare earth chlorides issued vapor at 998°C°. The resulting optical fiber preform was homogenous and had appreciable concentrations of Er 2 O 3 , and a Yb 2 O 3 concentration of 4.0 wt%.
• An optical fiber preform having a rare earth dopant was prepared by a combination of soot deposition and solution doping techniques. Soot deposition was conducted in the manner described in Example 1, but using the base glass deposition components and parameters set forth in the following Table II.
• soot deposition the core layers of soot were not subsequently sintered, nor was the tube collapsed. Rather, the soot preform was placed in a tubular container, and subsequently evacuated to lower than about 1 to 10 "1 Torr.
• the solution shown comprising ingredients set forth in the following Table III was introduced into the container under vacuum soaking the soot coated tube.
• the doping solution was poured away and the soot coated tube was dried at approximately 150°C to 250°C. The soaking procedure was repeated.
• the soot coated tube was then dehydrated, calcined, and sintered and collapsed, yielding a finished co-doped optical fiber preform.
• Dehydration using a stream of gas containing Cl 2 (delivered at 50 cc/min to 80 cc/min) and 0 2 (delivered at 1,000 cc/min), was conducted at 150°C for approximately 30 minutes, then at about 750°C to 800°C for about 2 hours.
• Sintering and collapsing was conducted in a single pass at a temperature in the range of about 1,960°C to 1,980°C with a gas stream comprising 0 2 and He (both delivered at 1,000 cc/min) passed through the bore of the tube.
• the core composition of the finished co-doped optical fiber preform comprised, as determined under standard Electron Probe Microanalysis, 2.5 wt% Yb 2 0 3 , 0.3 wt% Er 2 0 3 , and a silica content greater than 97 mol%.
• An optical fiber preform was prepared, using the base glass deposition components and deposition parameters set forth in the following Table IV.
• a glass forming vapor comprising the listed components was directed into the bore of a quartz tube 5 through outer delivery tube 240 of the multi-concentric delivery system 20 illustrated in Fig. 1.
• the high temperatures used in each pass resulted in the immediate deposition of a monolithic glass layer on the internal surfaces of the quartz tube 5.
• a stream of helium gas (having a flow rate of approximately 300 cc/min) was passed though aluminum chloride material, which was loaded and heated to about 120°C to 150°C in column 122 shown in Fig. 1.
• a stream of helium gas (also having a flow rate of approximately 300 cc/min) was passed through ytterbium chloride material, which was loaded and heated within the temperature range of about 910°C to 930°C (depending on desired concentration) in a rare earth chloride boat 32 positioned inside the central delivery tube 200.
• the resulting helium stream, laden with rare earth chloride vapor, was further diluted with a stream of helium gas (having a flow rate of approximately 700 cc/min) and directed into the bore of the quartz tube 5.
• Glass frits 202 and 222 were used to spread the A1C1 3 and ytterbium chloride streams into the base vapor stream which are then reacted under extremely high temperatures, i.e., in the range of about 1,800°C to 1,850°C, to form one or more layers of amorphous glass on the surface of the bore. Collapsing the tube resulted in the finished optical fiber preform.
• the core composition of the finished optical fiber preform was determined under standard Electron Probe Microanalysis.
• the data collected in mol% was identical to that retrieved from Example 1, which again are as follows: 98.4% SiO 2 , 0.65% A1 2 0 3 , 0.6% Ge0 2 , and 0.3% Yb 9 0 3 .
• a preform was prepared by the prepared by the process used in Example 3. However, instead of using extremely high temperature during deposition of the core layers to thereby form immediately amorphous glass, a comparatively lower temperature of about 1,650°C was used to deposit layers of soot. The layers of soot were then treated with 50 cc/min chlorine for about one to two hours to enhance water removal, then, sintered at approximately 1,980°C to 2,000°C to fuse the soot layer into a uniform glass layer. Collapsing the hollow tube resulted in a solid cylindrical optical fiber preform. The finished preform had similar composition as the preform of Example 3. The attenuation was as low as approximately 4 dB/Km at lasing wavelengths longer than 1 ⁇ m.
• Three optical fiber preforms (i.e., examples 3B to 3D, respectively) were prepared by the process used in Example 3. However, for example 3B, the ytterbium chloride material was vaporized at about 930°C, for example 3C, at about 950°C, and for example 3D, at about 980°C.
• Example 4 Upon analysis, it was observed that the core compositions in each of examples 3B to 3D substantially identical, but that ytterbium oxide concentrations increased from 3B to 3D, i.e., the rare earth oxide concentration increased as the temperature used for vaporization increased.
• the ytterbium oxide concentration in the optical fiber preform of Example 3C was greater than 3 wt%.
• a preform according to the invention was prepared using Nd-cyclopentadiene as dopant precursor.
• the base glass deposition components and parameters are set forth in the following Table V.
• a stream of helium gas (having a flow rate of approximately 300 cc/min) was passed through aluminum chloride material, the aluminum chloride material having been loaded and heated to about 120°C to 150°C in the column 122 of the VPD apparatus shown in Fig. 1.
• the resulting stream helium gas, laden with A1C1 3 vapor, was directed into the bore of quartz tube 5 through delivery tube 220.
• a stream of helium gas (also having a flow rate of approximately 300 cc/min) was passed through the rare earth compound Nd-cyclopentadienide (ND-CP 3 ), the ND-CP 3 material having been loaded and heated to approximately 230°C in column 124.
• a chloride boat 32 was not used in central delivery tube 200.
• Glass frits 202 and 222 dispersed the A1C1 3 and ND-CP 3 streams into a base glass vapor stream, the base glass vapor stream have been formulated as indicated in the table above and delivered into quartz tube 5 through outer delivery tube 240.
• the vapor laden helium streams arriving and merging in quartz tube 5 were reacted under extremely high temperatures (i.e., about 1,800°C to about 1,850°C) to form one or more layers of rare earth doped amorphous glass on the surface of tube 5.
• the tube was collapsed to provide a finished optical fiber preform.
• the core composition of the finished optical fiber preform was determined under standard Electron Probe Microanalysis.
• the neodymium oxide concentration of the preform was 1 wt%, the silica content of the preform was greater than 97 mol %.
• the core attenuation of a double clad fiber laser prepared from the preform in accordance with the methods disclosed in U.S. Patents Nos. 4,815,079 (Snitzer et al.) and 5,534,558 (Minus) was lower than 10 dB/Km at about 1,000 nm to 1,200 nm lasing wavelength.
• the slope of efficiency of the double clad fiber laser was greater than 50%.
• An optical fiber preform was prepared in the manner described for Example 2. However, higher temperatures were used to effect the deposition of the cladding layer, thus resulting in the relative immediate deposition of amorphous glass. To accommodate such higher temperature, slight modification were made to the base glass deposition components and parameters as shown the following Table VI .
• Example 2 All else was similar to Example 2, including the solution doping steps following deposition of core layers of soot. Regardless, the core composition of the finished optical fiber preform, as determined by standard Electron Probe
• Microanalysis was similar to that of the preform made in Example 2: i.e., 2.5 wt % Yb 2 0 3 and 0.4% wt% Er 2 0 3 , and a silica content greater than 97 mol%.

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