CN1307544A - Method and apparatus for manufacturing a rare-earth metal doped optical fiber preform - Google Patents

Abstract

A method and apparatus is disclosed for the manufacture of an optical fiber preform having incorporated therein a comparatively high concentration of rare earth metal 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 metal dopant material are accomplished through a ''hybrid vapor processing'' (HVP) method or a ''hybrid liquid processing'' (HLP) method, either being practiced in combination or independently of one another. The HVP method involves the vaporization of a rare earth metal halide by the exposure thereof to a sufficiently elevated temperature, independently, or contemporaneously with the transport of the resultant rare earth metal halide laden vapor, into a glass-forming oxidation reaction zone on a flowing stream of essentially an unreactive inert gas, such as helium. According to the HLP method, a first amount of rare earth metal dopant is provided according to the HVP method and/or other vapor source of rare earth metal dopant which is mixed with glass-forming vapors to form a deposited soot layer on the internal surface of a glass tube. The soot-deposited tube is then impregnated with a dopant solution comprising, a second amount of rare earth metal dopant. The tube is then thermally collapsed resulting in an optical preform with an enhanced amount of rare earth metal dopant incorporated at a comparatively high concentration. The apparatus comprises means, such as tubes, for introducing the rare earth metal dopant as a vapor, formed from a solid state form of the dopant, into the main glass deposition tube separately from glass-forming material vapors and oxygen for the reaction within the main tube.

Description

Method and apparatus for manufacturing optical fiber preform doped with rare earth metal

RELATED APPLICATIONS

This application claims priority from provisional application 60/091,290 at 30/1998 and provisional application 60/091,154 at 30/6/1998.

Technical Field

The present invention relates to a method of making an optical fiber preform doped with a predetermined high level of rare earth dopant, and in particular, where the rare earth dopant is present in a relatively high concentration and the cross-sectional geometry of the preform promotes good mode interference.

Background

The optical fiber is a superfine optical conduit. Light is drawn into one end, propagates forward through the fiber within the fiber, whether the fiber is straight or curved, and finally exits the other end. By drawing light into the fiber in a predetermined manner, a large amount of information can be communicated almost instantaneously (i.e., at the speed of light) over a large bandwidth across a remote geographic distance. The availability of optical fibres is undisputed because of their thinness, quickness and robustness.

The variety, form and complexity of optical fiber structures are increasing, but almost all optical fibers have a main basic structure of a light guide core and an outer cladding. The refractive indices of the core and the cladding are adjusted during the manufacturing process so that the cladding has a lower refractive index than the core. When light is drawn into the core, encountering the difference in refractive index at the core/cladding interface, in an optical phenomenon known as "continuous internal reflection", the light is "folded back", with very little loss of light within the core, and thus propagates continuously along the optical fiber.

In manufacture, optical fibers are typically drawn from an optical fiber preform having the same core-cladding arrangement as the final optical fiber in cross-section, but several orders of magnitude larger in diameter than the optical fiber. One end of the preform is heated in a furnace to a flexible plastic consistency and then longitudinally drawn into a fiber of the desired core/cladding size.

In the preform fiber manufacturing industry for transmission fibers (as opposed to active fibers, i.e., single mode or double clad fibers with rare earth doped cores), high speed production techniques have been developed that reduce cost while providing high quality optical fibers by using chemical deposition methods where the components are fed as vapors into a horizontally rotating refractory tube to form one or more internal glass layers on the inner surface of the tube. For example, MacChesney et al, patent 4,909,816 and its allied patents 4,217,027 and 4,334,903, disclose a process known as Modified Chemical Vapor Deposition (MCVD) which distinguishes it from the general semiconductor-type CVD process and the CVD processes used in the past to produce glass preforms. The above patents discuss MCVD to augment the "soot" or Outside Vapor Deposition (OVD) process described in us patents 3,775,075 and 3,826,560. The' 816 patent is directed to establishing a more efficient homogeneous reaction in which the reaction products of the gas phase are entrained in the gas stream within a refractory tube to form glass precursor particles which are then deposited on the inner surface of the tube downstream of the heating zone or heat source. As the heating zone moves, the deposited particles then coalesce into a transparent glass layer on the tube surface. This is different from the CVD used to make glass preforms, which explained a heterogeneous reaction that first forms glass particles on the inner surface of the refractory tube and the soot layer formed is sintered to form a glass layer, or directly forms a glass layer, which is deposited to form a monolithic glass layer, as opposed to first forming glass particles in the glass refractory tube. The homogeneous reaction of MCVD is realized by raising the temperature of the reaction zone in a heat transfer zone. The MCVD method is superior to the OVD method in that hydrogen-containing components, water vapor and other impurities in the deposited glass layer are eliminated. A brief description of MCVD is found in U.S. Pat. No. 4,257,797 to Andrejco et al, and is described in detail in "fiber communication" at Vol.1, pages 1-64 (fiber Fabric, Tingye Li, 1985, Academic Press, Inc.).

According to the' 861 patent, a higher productivity of the glass preform is achieved by a continuous, uninterrupted process followed by a large-scale production of optical fibers by fiber drawing, said process comprising: the reaction temperature for forming the glass layer is increased, the rotation speed of the tube is increased, the deposited glass layer is sintered, the influence of hydration pollution on the deposited glass layer is reduced as far as possible, and the tube is rotated and thermally sintered into a preform. While this high speed processing method is well suited for the manufacture of transmission fibers, it is not suitable for the manufacture of active optical fibers, particularly when high levels of rare earth dopants or co-dopants need to be doped in one or more deposited layers on the inner surface of the refractory tube. Active fibers are used as fiber gain media for signal amplification in fiber lasers, consisting of single-mode or double-clad fibers with a core doped with 4f rare earth elements (i.e., lanthanides of atomic number 57-71), such as erbium or ytterbium doped or co-doped. By selecting a particular concentration and/or combination of rare earth dopants, the absorbance of the core for light of a particular wavelength can be determined as desired. A suitably doped core, with a suitable cladding structure, in combination with a suitable pump source, may provide a basis for excitation and/or amplification of light. An optical fiber having the above-described optical amplification function is highly desirable in view of, for example, signal amplification requirements in optical fiber telecommunications. Unfortunately, rare earth doping, especially high concentration doping, in the core is not easy to perform.

There have been many methods for fabricating rare earth doped optical fiber preforms. For example: miller et al, 26/2/1985, U.S. Pat. No. 4,501,602; U.S. patent 4,616,901 to MacChesney et al, 10/1986, 14/l; U.S. patent 5,236,481 to Berkey, 8.1993, 17.d; U.S. patent 5,609,665 to Bruce et al on 3/11/1997; us patent 4,501,602, Miller et al, 2.2.1985; U.S. patent 4,826,288 to Mansfield et al, 5.5.2.1989. Nevertheless, it is difficult in current practice to dope high concentrations of rare earth dopants, particularly the commonly used rare earth element neodymium (Nd), with a limited overall doping level.

The problems include: the most common preform fabrication method, MCVD, requires the generation of a rare earth dopant-rich vapor and deposition of a layer. In current practice, only a low vapor pressure is generated and consequently only a low concentration of rare earth dopant can be doped in the end. The inability to achieve high rare earth dopant concentrations has resulted in the inability to produce optical fibers having small numerical apertures, low core attenuation, and high pump power absorption, all of which are desired indicators in fiber laser and amplifier design.

In addition, in particular, in the case of a fiber laser, even if an appropriate optical fiber preform is produced, the laser efficiency of an optical fiber drawn therefrom is otherwise deficient. The performance of a fibre laser, as in any active or nonlinear waveguide, is closely related to the efficiency with which the pumping radiation can be absorbed by the active substance in the core. In the earliest fiber lasers, the bulk of the radiation pumped into the fiber was unable to pass through the core and therefore did not contribute to the lasing effect of the core. To this end, various cross-sectional fiber geometries have been successfully developed, particularly with respect to the cross-sectional geometry of the inner cladding of a double-clad fiber, which can affect the manner in which internal reflection occurs, with a higher probability of interaction of the core with light propagating along the inner cladding, crossing and being absorbed within the doped core. See, for example, U.S. Pat. Nos. 4,815,079 to Snitzer et al, 3/21/1989; and U.S. patent 5,533,163 to m.h. muendel, 7/1996, 2/h. However, other additional manufacturing steps are required in the formation of the preform according to the above known design of the optical fiber. Any improvement that reduces these additional steps while enhancing light scattering in the inner cladding and thus enhancing absorption in the core is desirable.

Summary of The Invention

According to the inventionThe method provides a method for manufacturing an optical fiber preform with a high content of rare earth dopant, and the preform can be drawn and processed into an optical fiber with small numerical aperture, low core attenuation rate and high pump power absorption. In practice, high concentrations of rare earth dopants are achieved by using what we call "mixed vapor phase processing" (HVP) or "mixed liquid phase processing" (HLP), either alone or in combination. The method of the present invention can be used to form various types of optical fiber structures with high optical uniformity, including glass preforms for single mode fibers and double clad fibers. The HVP method comprises: the solid rare earth metal halide is vaporized at a sufficiently high temperature while the rare earth halide vapor formed is carried with an inert gas (e.g., helium) into an oxidation reaction zone located within the bore of the hollow refractory tube. While introducing a glass-forming species (e.g., SiCl) into the reaction zone₂) Of (2) is added. By adjusting the temperature of the reaction zone, one or more layers of soot or a single sintered glass layer can be deposited directly as in patent 4,909,816 and other earlier patents. A soot layer is deposited on the inner surface of the refractory tube by an oxidation reaction of the rare earth halide vapor and the glass forming material vapor. The vacuum tube is then collapsed into an optical fiber preform.

As used herein, "soot layer" refers to a deposited layer having a high porosity and not yet fully sintered into a glass or amorphous layer, and thus, it does not have the optical clarity, optical properties and uniformity of a single glass layer formed after high temperature sintering.

An important feature of the HVP process is that the rare earth dopant delivery system employed provides the rare earth vapor from the solid state prior to vapor deposition (VPD) in admixture with the introduced oxygen or glass forming material oxide. In other prior art, such as patent 4,909,816 and its allied patents, the rare earth vapor is almost immediately contacted with oxygen or an oxide. We have found that this has a positive and complex effect on the uniformity of the components within the soot deposition layer deposited on the inner wall of the refractory tube. It has not been possible to uniformly dope the rare earth component continuously and reproducibly, let alone as a leveling agent, into an intermediate. The HVP process of the invention forms a layer with a high degree of optical uniformity by uniformly and reproducibly incorporating higher concentrations of rare earth and/or intermediate components using the novel delivery system of the invention. By optical uniformity we mean that the resulting deposited sintered monolithic glass layer has a deposited frit irregularity of less than 2 μm in width. Beyond this value, it is believed that the glass frit is not fully reacted and fully converted to an intermediate or the like, and the homogeneously mixed glass component and rare earth dopant are homogeneously doped into a monolithic amorphous glass layer, and thus are non-uniform and unacceptable.

The HLP process includes depositing one or more layers containing a first portion of a rare earth dopant on an inner surface of a refractory tube to form one or more soot layers. The layers are deposited at a temperature such that a porous frit layer is formed without conversion to a continuous monolithic glass layer. This step can be carried out by the HVP method or by the standard VPD method known in the art. After both methods, the soot layer deposited refractory tube is then transferred from the preform frame to a dopant solution formulated with a second portion of dopant for impregnation. And the melting-resistant tube returns to the precast rod rack, the impregnated doped layer is heated and sintered, and then the doped layer is sintered and shrunk, so that the final content of the rare earth doped material in the obtained optical fiber precast rod comprises the sum of the first part and the second part of the rare earth doped material.

The HVP or HLP process produces an optical fiber preform that can be stretched for its original geometry or trimmed prior to being drawn into an optical fiber to alter the optical properties of the glass preform, such as the light scattering mechanism. Mechanical grinding or chemical processes can be used to form flat or concave surfaces on at least one longitudinal side of the preform, which is well suited to changing the geometry before the fiber is drawn, for example, forming an outer cladding for drawing a double clad preform or forming a jacket for drawing a single mode fiber. More than one plane may be created on the preform, for example on opposite longitudinal surfaces of the preform.

As noted above, it is a principal object of the present invention to provide an improved method and apparatus for fabricating a high rare earth dopant content optical fiber preform wherein the defined total doping concentration of the glass fiber preform is sufficient to provide an optical fiber produced from such preform having a low numerical aperture.

It is another object of the present invention to provide a method for fabricating an optical fiber preform wherein high levels of rare earth dopant incorporation are achieved by vaporizing solid rare earth feedstock material near the region where the glass forming material is deposited on the inner surface of a rotating refractory tube. More than one source of rare earth can be used to provide different rare earth dopant-containing vapors to the reaction zone within the refractory tube.

It is another object of the present invention to provide a method for fabricating an optical fiber preform, in which a rare earth dopant is doped by first forming a rare earth vapor deposition soot layer, followed by rare earth solution doping.

It is also an object of the present invention to provide a method for fabricating an optical fiber preform characterized in that the vapor deposition process employs a method that reduces the premature occurrence of particle-forming oxidation reactions, thereby being more suitable for forming a uniform monolithic glass layer.

It is also an object of the present invention to provide a method of fabricating an optical fiber preform that is less susceptible to contamination by water, which is often found to be associated with the inherent moisture sensitivity of halogen-based dopants commonly used in preform fabrication (e.g., aluminum chloride, rare earth chlorides, etc.).

It is also an object of the present invention to provide a method for fabricating an optical fiber preform, which uses cyclopentadienyl rare earth (CP3) and/or its derivatives as a dopant.

The present invention is also directed to a method of manufacturing an optical fiber preform, comprising the steps of: rare earth halides, such as rare earth chlorides, in solid form are vaporized at a sufficiently high temperature and carried with a non-reactive inert gas stream, such as helium, to an oxidation reaction zone, to which glass-forming substance vapors are introduced.

It is also an object of the present invention to provide a method of trimming an optical fiber preform by changing the geometry of the preform to change its optical properties.

Several preferred embodiments of the present invention are described in more detail below, as illustrated in the accompanying drawings. These and other features and advantages of the present invention will be further apparent.

Brief Description of Drawings

FIG. 1 shows a vapor deposition (VPD) apparatus for depositing a soot layer or a single amorphous glass layer on the inner surface of a supported hollow refractory tube in the practice of the present invention.

FIG. 2 shows a cross section of an optical fiber preform according to one embodiment of the present invention, which is then collapsed into a glass preform.

FIG. 3 shows drawing of a glass preform into an optical fiber.

FIG. 4 shows the attenuation versus wavelength for an optical fiber prepared according to one embodiment of the present invention.

FIG. 5A shows a cross-section of a double-clad optical fiber prepared according to one embodiment of the present invention.

FIG. 5B shows a cross-section of another double-clad optical fiber prepared according to one embodiment of the present invention.

FIG. 6 shows a rare earth chloride container or boat for use in one embodiment of the present invention.

FIG. 7 shows a first modification of the glass tube conveying system shown in FIG. 1.

FIG. 8 shows a second modification of the glass tube conveying system shown in FIG. 1.

Detailed description of the preferred embodiments

The applicability of the present invention is discussed below in terms of fabricating a preform that can be drawn into a double-clad optical fiber, but is equally applicable to fabricating preforms for other types of optical fibers, including single mode or multimode optical fibers. In particular, the methods of the present invention relate to, for example, the strengthening of rare earth doping in the core, and thus, embodiments of the present invention are particularly well suited for use as single-clad and double-clad fibers for active gain media such as fiber amplifiers or fiber lasers. For purposes of illustration, fig. 5A and 5B show representative examples of fiber gain media. However, the principles of the methods used herein are also applicable to the addition of other components in addition to rare earths within glass fiber preforms, which may also be added using HVP and/or HLP methods. One example is to increase the incorporation of phosphate in a glass-forming layer deposited on the inner surface of the tube in addition to or in place of the rare earth component.

As shown in cross-section in fig. 5A and 5B, a double clad fiber for a high power fiber amplifier or laser has a core 14, an inner cladding 5, an outer cladding 40, or optionally a protective outer jacket 50. Alternatively, the double-clad fiber may have another inner cladding 12 with a thinner cross-section, forming an interface between the core 14 and the cladding 5. The portion of the optical fiber that is drawn from a glass preform, such as preform 10 "in fig. 2, is labeled 10 in fig. 5A and 5B.

When the refractive index n of the core 14₁Greater than the refractive index n of the inner cladding 5₂The propagating radiation may be primarily confined within the core 14 by total internal reflection. The inner cladding 5 has a refractive index n₂With an inner cladding 5 and a lower refractive index n₃Acts as a waveguide by internal reflection of radiation at the interface between the outer cladding layers. The purpose of the inner cladding is to confine the emitted radiation within the inner cladding so as to repeatedly cross the core 14 as it propagates longitudinally along the fiber. In each such crossing over with the core 14, a portion of the radiation (also called pump light) is absorbed by the active gain dopant (e.g., rare earth dopant) doped in the core 14. The fiber is typically tens or hundreds of meters long, so that a large number of such core interactions can occur, whereby as much pump radiation as possible is absorbed by the core.

While current double clad optical lasers work well, it has been found that this good effect can be further improved and/or made more economical as follows: 1) increasing the concentration of active gain dopants (e.g., rare earth dopants) in the core, and/or 2) designing simple fiber geometries to promote light absorptive interactions within the fiber single mode core at moderately high frequencies.

Although ultimately the end product of a double clad fiber laser is to be obtained, embodiments of the present invention are primarily directed to the design and manufacture of an optical fiber preform 10 "from which the central optical strand 10 of the fiber, i.e., the strand comprising the core and inner cladding structure, can be drawn. As is well known in the art, the structure and combined construction of the rod preform 10 ", although having a substantially reduced cross-sectional diameter, can be accurately converted into a much longer filamentous fiber 10 by stretching the preform 10".

We have found that active gain rare earth dopants can be doped at high concentrations in optical fiber preforms by: delivering a high vapor pressure rare earth halide that is substantially free of oxides and moisture to a glass forming reaction zone (i.e., a mixed vapor phase process or HVP process); alternatively, the dust deposition is combined with solution doping (i.e., mixed liquid phase processing or HLP process) originally. The above two methods can be used alone or in combination.

According to the HVP process of the invention, oxidizable rare earth halides, such as rare earth chlorides, having the desired high vapor pressure are produced directly by vaporizing solid rare earth halides in an oxygen-free, moisture-free, high temperature environment. The advantages of this streamlined process are high efficiency, uniformity and high yield, which are difficult to achieve with the prior art (e.g., U.S. patent 4,616,901 to MacChesney et al) using a multi-step process for generating oxidizable rare earth vapors.

As shown in FIG. 1, the vapor deposition (VPD) apparatus of the present invention employs a newly designed transport system 20 comprising multiple concentric quartz glass tubes 200, 220 and 240 equipped with forward and reverse flow regulators 202 and 222, such as gas permeable quartz glass plugs or plugs of other such inert gas permeable materials, which close the output ends of the respective inner concentric transport tubes 200 and 220. In essence, the multiple concentric delivery system 20 of the HVP apparatus of the present invention allows for the regulated delivery of vapors of various gaseous materials to the inner cavity of the quartz tube 5 without contamination by oxygen reflux provided by the outer concentric delivery tube 240. In this manner, uncontaminated rare earth dopant vapors provided by transfer tubes 200 and 220 are first reacted with other gases and glass-forming component vapors delivered from outer transfer tube 240 in a definable reaction zone 5B longitudinally of tube 5 by heater 340 to form one or more layers of monolithic glass or other particles or soot on inner surface 5A.

The VPD equipment can be flexibly applied to various vapor deposition processes by slightly changing the VPD equipment. For example, the assembly 42 may be shut down, isolated, removed, or otherwise "taken off-line" while the HVP process is being performed, and a solid rare earth halide, such as a rare earth chloride, is loaded into the boat 32 located within the central transfer tube 200. Alternatively, to further enhance the rare earth incorporation into the glass or soot deposit, the rare earth vapor source 42 may be activated "on-line," in combination with the rare earth chloride in the vaporization boat 32. However, it must be understood that both of the above rare earth chloride supplies may be used independently.

One of the important features of the present invention is the use of a rare earth chloride boat. First, it is noted that the use of solid rare earth raw materials and their subsequent conversion into high vapor pressure liquids is not new in itself. For example, in U.S. patent 4,666,247 to MacChesney et al, the silica quartz lumen 24 contains rare earth powder or liquid (NdCl)₃) Because the output end of the tube 24 is protected from impurities entering the tube 24 by the quartz wool 25, the powder or liquid is heated to about 1000 ℃ by the heater 18 in a substantially oxygen-free environment. However, the difficulty with this approach is that as the rare earth source material is consumed as the process progresses, the surface area of the rare earth mass, rare earth heap or irregular form of rare earth changes over time and, as a result, the concentration of rare earth dopant incorporated in the process also changes over time. This problem is also true for liquid rare earth ponds that do not have a constant boundary. The total amount of rare earth in vapor form varies with temperature, vapor pressure at a temperature, and exposed surface area of the rare earth component. Taking the sintered lumen shown in patent 4,666,247 using powder or liquid rare earth as an example, the exposed surface area changes constantly as the solid or liquid component dissipates, reducing the total surface area. Therefore, in order to control the time uniformity provided by the rare earth vapor,a movable boat 32, shown in fig. 6, is used with a chamber 34 that is constant in width 38 and length 40 as the rare earth source material 36 is consumed over time. As a result, a constant vaporization surface area is maintained throughout the period of time that the rare earth source material is present in the boat 32 until another boat is exchanged, which is not possible in patent 4,666,247. Rare earth source materials 36 are loaded into the chamber 34 of the quartz boat 32 so that the vapor deposition process can be run multiple times and the vaporization thereby performedThe exposed surface is kept substantially constant, thus ensuring a "batch-to-batch" uniformity of the preparation.

To prepare the boat 32 for the conveyor system 20, the rare earth powder may be melted within the chamber 34 to form solid rare earth chlorides having two- dimensional dimensions 38 and 40 integral with the boat 32. This can be done, for example, in an inert atmosphere (e.g., helium) that does not contain oxygen at about 900 ℃. In the manufacturing process, first, the boat is placed in a sealed chamber containing a rare earth (e.g., Yb) powder. Then, the cabin is charged with halogen (e.g., Cl)₂) A gas and an inert carrier gas (e.g., He). Raising the temperature in the cabin to about 500 ℃, and discharging water vapor and oxygen through an exhaust system in the cabin according to the following reaction formula:

this form of rare earth is very good because it provides a low vapor pressure without oxygen. The inclusion or presence of oxygen in the rare earth will increase its vapor pressure.

The temperature of the boat was further increased to about 900 c to melt the rare earth powder and provide exposed surfaces having two- dimensional dimensions 38 and 40 as shown in fig. 6. The resulting rare earth boat is then cooled and transferred directly into the transfer tube 200 to avoid prolonged contact with the oxidizing atmosphere. The flow/anti-reverse flow plug 202 is removed, the boat 32 is placed into the downstream end of the tube, and this plug is then re-installed into the end of the tube, or replaced with a new plug if the old plug becomes severely plugged by a large amount of oxide deposited on the outer surface. After the filter plugs are installed and the conveyor system 20 is reassembled, the introduction of the inert gas stream is initiated and the low temperature (500 ℃) heating by the heater 360 drives off the water and oxygen absorbed by the boat as it is transferred into the system 20. Then, in order to generate and deliver the rare earth vapor from the boat 32, the heating temperature is increased to 1000 ℃, and the generated high vapor pressure rare earth vapor is carried by the inert gas helium. Thus, the method provides a source of low vapor pressure solid rare earth components that is free of any moisture or oxygen contamination.

The used boat can be refilled, that is, the rare earth raw material is added into the residual rare earth raw material in the boat, the refilled boat is dehydrated and deoxidized, and then the boat and the rare earth raw material are melted into a whole.

As shown in FIG. 1, an inert carrier gas, such as helium, is injected from a source not shown through mass flow controllers 112 and 114 into reservoirs, feed tubes or columns 122 and 124, respectively. Mass flow controllers are well known in the art as flow regulated gas sources. Columns 122 and 124 serve as a source of various vapor-laden gases. In the embodiment shown, the pillars 122 are intermediates (leveling agents) such as AlCl₃A source of vapor, which is an intermediate that changes the refractive index. Other intermediates or mixtures thereof may be used, such As halide vapors of Ga, In, As, and/or Sb. These intermediates allow for uniform mixing of the components during the glass forming process. For simplicity, only one intermediate source is shown, but it will be understood that other intermediate sources orWhich are combined while being mixed with an inert carrier gas delivered by the controller 16. Column 124 is a rare earth chelate vapor containing rare earth. As shown, a particularly useful rare earth-containing vapor is a "rare earth" -cyclopentadienyl (RE-CP)₃) Steam, e.g. Yb (C)₅H₅)₃Or Er (C)₅H₅)₃Or mixtures of such rare earth vapors. The chemical formula of the rare earth-cyclopentadienyl compound is RCP₃Wherein CP₃May be a hydrocarbon (C)₆H₅)₃R is a rare earth element such as, but not limited to, neodymium (Nd), ytterbium (Yb), erbium (Er), thulium (Tm), holmium (Ho) and samarium (Sm). The preferred rare earth chelate vapor source 124 is cyclopentadienyl neodymium vapor because it can be oxidized to useful active gain Nd-cyclopentadienyl dopants at high temperatures above 1000 ℃.

Columns 122 and 124 are suitably heated to a temperature of up to about 220 c. Of course, different rare earth materials may require different temperatures because different rare earth compounds have different vapor pressures.

As shown in fig. 1, the vapor of the feed from columns 122 and 124 is provided to the transport system 20 through three- way stopcocks 132 and 134, respectively.

An additional inert carrier gas, such as helium, may be provided by the controller 110 through the tube 144 and the stopcock 130 to adjust the amount of inert gas provided to the transfer tube 200.

The stopcocks 132 and 134 have separate bleed intermediates (AlCl)₃) And an outlet for rare earth chelate vapor, forming equilibrium gas flow and temperature conditions in columns 122 and 124, and then introducing the vapor into a transfer tube leading to a CVD reactor comprising rotating tube 5 and its heated zone. Independently controllable stopcocks 132 and 134 are used to introduce the desired vapor mixture into the reactor system at the beginning of the rare earth-containing soot deposition and to shut off the vapors after the desired deposition is complete.

The feed vapor is carried by an inert carrier gas, such as helium, through mass flow controllers 116 and 110, respectively, into carrier tubes 142 and 144. Such delivery tubes are well known in the art. For example, the transport tubes 142 and 144 are 0.63cm diameter Teflon @tubingwrapped with 3.8cm copper tubing. The temperature of the transport tubes 142 and 144 is maintained at a temperature high enough so that the transported feedstock vapors do not condense. In this regard, thermocouple temperature sensors may be used to control the temperature variation within the transport tubes 142 and 144 to within 2-3 ℃. This is sufficient to ensure accurate control of the temperature and vapour pressure of the feedstock vapour being carried within the tube, thereby ensuring its stability.

The transport tubes 142 and 144 are connected to concentric transfer tubes 220 and 200, respectively, of the transfer reaction system 20. A stream of unheated mixed gas (source not shown, but well known in the art) enters the outer concentric delivery tube 240. In the example shown in FIG. 1, this mixed gas contains He and O₂、Cl₂、SiCl₄、GeCl₄、POCl₃、BBr₃、SF₆And CF₄. As known in the art, the other gas may be, for example, SiO₂、P₂O₅、AlO₃MgO, CaO or K₂And O. Typically, active fibers employ Al or P as the index changing component, but these elements are generally not used in the manufacture of transmission fibers. These gases may be introduced into the transport tube 240 as a mixture because they do not react at the temperature of the tube 240 at the upstream section of the transport system 20 and at the reaction zone 5A. The concentric transfer tubes 200, 220 and 240 have heating elements 290 in the center for the vapor as it enters the tubes through the respective tube tailsIt is initially heated and can prevent substantial premature thermal decomposition of these component vapors on the tube walls.

As previously described, downstream of the inner transport tube is a holding container 25 or boat 32 for holding solid elements or compounds to be vaporized at high temperature in the high temperature zone 362 (including the boat 32 within the tube 200). The high temperature zone 362 is heated by another heating element 360, which coaxially surrounds the concentric tubes 200, 220, 240 and the refractory tube 5 of the system 20. Any type of conventional heater may be used, for example, a radio frequency (rf) coil wrapped around a concentric tube as the heater, with the boat 32 being made of a radio frequency radiation highly absorbent material. One example of the use is a coaxially surrounding heating element 360 comprised of nichrome wire wrapped around a small bore aluminum tube, which has been found to be well suited for performing the task of vaporizing the rare earth elements in the boat 32. Because of the concentric arrangement, the innermost transfer tube 20, of transfer tubes 200, 220, and 240, is the closest to the thermal center of heating element 360 and is at the highest temperature, and the outermost lumen 240, which is furthest from the thermal center of heater 360, is at the lowest temperature. Thus, when the rare earth mixed vapor is introduced from the source 42 through the central transfer tube 200, it encounters the high temperature of the 362 region, e.g., 800-. The temperature used provided by the annular heater 360 depends on the desired concentration of the rare earth dopant in the glass layer to be deposited because the vapor pressure generated on the rare earth chloride boat 32 depends on the temperature of the heater used. The generating system 20 is coaxially inserted into the hollow tube 5 and connected thereto by, for example, a rotatable hermetic seal, so that the hollow tube 5 can be rotated about its axis at a desired rpm during deposition of the glass layer on the tube surface 5A. The rotation speed used in the present invention is about 40-60rpm, generally about 50 rpm. The rotation may be performed using a lathe or other methods known in the art. In the high speed production of preforms described in patent 4,909,816 and its equivalent, the lathe is rotated at about 100 rpm. Downstream, after the vapors leave the transport system 20, they enter the reaction zone 5A, the temperature of which is raised by a gas-fired belt burner 340, which can be moved back and forth transversely along the length of the tube 5, as indicated by arrow d, of course, without the extent of the transport system enclosure 20 within the tube 5. Thus, once the gaseous species are mixed in the 5B region of hollow tube 5, a chemical reaction occurs as the reactant vapor traverses the resulting high temperature zone 342 by burner 340. Rare earth oxide particles, such as cyclopentadienyl Nd, of one of the products are mixed with other elementary glass oxide particles into a dust layer, which is deposited on the walls of the hollow refractory tube 5.

The ribbon burner 340, which is movable axially back and forth (d-direction), is typically constructed of a slotted quartz or metal tube. Into which H is introduced₂Ignited, forming a flame, and heating the tube 5 to a temperature required for dust formation and deposition on the surface 5A. In operation, as is known in the art, a dust layer is typically deposited in an area several inches downstream from the reaction zone SB. To deposit a dust bed along the tube 5, the ribbon burner 340 is moved downstream along the length of the tube 5. The burner 340 is repeatedly moved downstream along the length of the tube 5, thus forming a dust layer or amorphous glass layer on the surface 5A (depending on the reaction temperature generated by the burner).

Simultaneously with, immediately after, or after the final vapor deposition, the quartz tube 5 is sintered to a final optical fiber preform. The sintering is carried out by applying very high temperatures to the quartz tube 5, for example above the temperature at which the glass is deposited and sintered. Good collapsing effect is obtained by pivoting the quartz tube 5 as the ribbon burner 340 moves upstream along the length of the tube 5.

Performing HVP with a VPD apparatus includes retrofitting a conventional VPD apparatus for MCVD, specifically including using the rare earth boat 32, preventing contamination (e.g., premature oxidation) of the vapor prior to introduction into the reaction zone 5B with the regulators 202 and 222, and generating helium gas through the elements 110 and 116 as an inert carrier gas carrying dopant vapor. However, to obtain certain advantages in the performance and results of the HVP, additional points of the regulators 202 and 222 and the rare earth boat 32 need to be further considered.

When all of the component vapors exit tubes 200 and 220 of delivery system 20 into the lumen of hollow tube 5, they must first permeate through quartz glass wool gas plugs 202 and 222. The quartz glass wool 202 and 222 is gas permeable and allows the carrier gas carrying the rare earth chloride vapor to be dispersed into a fine, uniform gas stream through the filter plugs 202 and 222. In one mode of operation, the rare earth vapor stream within the transfer tubes 200 and 220 is first passed through a quartz wool plug 202 and mixed with an intermediate (leveling agent) such As an Al, Ga, In, As, and/or Sb halide vapor downstream of the transfer tube 220. They are then mixed with halide (e.g., halide of silicon, germanium, boron and/or phosphorus) vapors of glass-forming components immediately after entering tube 5 through quartz wool plug 222. The use of plugs 202 and 222 makes the composition of the soot or monolithic glass deposition layer more uniform than conventional MCVD, especially with respect to the doping of the deposition layer with rare earth dopants.

While the inventors do not wish to be bound by any particular theory to explain the present invention, it is believed that the plugs 202 and 222 function to prevent premature particle formation during uniform deposition and thus may first deliver a uniform vapor from the internal delivery system and then encounter the high temperatures generated by the heat source 340. This facilitates uniformity of the mixed vapor with the dopant vapor, resulting in uniformly doped particles. Furthermore, it is also believed that the use of plugs 202 and 222 and the downward flow of gas substantially prevents the backflow of oxygen into the transfer tube 200 where the HVP process of the invention directly produces rare earth chloride vapors. The oxygen reflux causes premature oxidation of the rare earth chloride vapor to form particles. If the particles form prematurely, they enter the reaction zone 5B, continue to grow and eventually deposit downstream, and when the deposited layer is sintered, they become part of the resulting glass that has properties different from the surrounding areas, a defect known in the art as "blistering". The presence of "bubbles" severely reduces the optical efficiency of the drawn optical fiber, and if it is a bubble such as a soot particle that is entrained with air, the resulting glass preform may be unusable for drawing into an optical fiber.

The HVP process of the present invention uses the delivery system 20 of the present invention to provide a glass-forming substance in a more uniform manner, particularly to achieve higher sustained rare earth dopant concentrations to form one or more glass layers within a glass tube or refractory tube that are not possible with the MCVD process. In operation, the HVP process begins by at least partially vaporizing the solid rare earth chloride in the vessel, i.e., boat 32, of the VPD apparatus shown in FIG. 1 at a sufficiently high temperature to continuously form a rare earth chloride vapor of suitable vapor pressure in the substantial absence of oxygen and moisture. The rare earth chloride vapour is carried by the inert helium gas stream to a reaction zone 5A located within hollow tube 5 and into which is simultaneously admitted the vapour of a material capable of forming glass at elevated temperatures. Then, the temperature of the reaction zone 5A is raised, and at least one layer of dust is formed on the surface 5A of the tube 5 from the vapor of the rare earth chloride and the vapor of the glass forming substance. The hot zone 342 moves downstream several times (but not necessarily downstream) to form a layer of powder. Unlike the formation of monolithic glass layers, the soot deposition process is carried out using a hot zone temperature of about 200-300 deg.C, which is lower than the MCVD process. The direction of movement of the hot zone 342 that causes the reaction and deposition may be unidirectional or bidirectional upstream and/or downstream. In HVP processes, deposition is typically performed while moving downstream. In the HLP process, which will be discussed below, the deposition is generally carried out while moving upstream, since moving upstream will form a layer of unsintered dust with a higher porosity, which can be used for the absorption of the rare-earth liquid in the next step. It should be noted that patent 4,909,816 is deposited in a downstream direction, forming a two-step operation in which sintering is carried out simultaneously with the formation of the soot deposit layer at a relatively high temperature, followed by immediate collapsing to rapidly form the glass preform. However, in the HVP process for forming a preform for producing an active optical fiber, 3 steps are used in total, wherein the first step is the deposition of soot layer, the second step is the sintering of soot layer, and the third step is the collapsing of glass tube. Preferably, further steps are interposed between the first and second steps, including dehydration and deoxidation of the powder layer before firing, or solution doped absorption of the dust after dehydration and deoxidation of the powder layer. In this case, a lower temperature is used in the deposition step to obtain a soot-like layer, i.e. a higher porosity, which patent 4,909,816 would avoid in order to produce preforms for transmission optical fibers with high yields.

Therefore, in the HVP process, in a preferred form of operation, first, a powder layer is formed on the inner surface 5A of the tube by moving the hot zone 342 several times, and then, SF is allowed to flow₆Or Cl₂Flowing through the tube to blow away the deposited layer of dustWater content (H) of₂O) and O₂(moisture is driven off in the form of hydrogen chloride) and, in a third step, the dehydrated powder layer is sintered at a higher temperature.

Finally, the glass tube 5 can be immediately shrunk at a higher temperature, for example, using the impregnation or absorption step of the rare earth solution (see below) in the inventive HLP process, followed by dehydration to drive off any residual moisture in the soot layer, and then sintering the soot layer into individual glass layers, depending on the intended purpose.

In the foregoing, reference is made to patent 4,666,247 for the rare earth chloride boat 32. It must be noted that an advantage of the present invention is the provision of a plurality of pre-mix tubes wherein the innermost tube can be formed from the rare earth component in a solid state with the rare earth component along with an inert carrier gas, but not containing AlCl as in patent 4,666,247₃And the like, so that the vapor pressure can be increased to obtain rare earth vapor. On the other hand, in the embodiment of fig. 1, the gasification of the solid rare earth is carried out under low vapor pressure in the presence of an inert gas but without any intermediate affecting its vapor pressure, and the rare earth vapor formed is then mixed with the intermediate in the second tube 220 via the filter plug 202. The low vapor pressure developed at the boat 32 is controlled only by the inert gas and the temperature of the boat 32.

It should also be noted that the use of quartz wool in the chamber 24 of patent 4,666,247 to create an oxygen-free atmosphere is not a good method because the rare earth chlorides readily react with the quartz wool to form oxides of the bulk material, and thus the rare earth chloride mixture reformed from time to time will not be the same. If a gas permeable glass plug is used, since this material is highly inert, although oxides may form on the outer surface of the outermost glass wool plug 222, this does not significantly affect its gas permeability, and therefore, the production of large quantities of glass preforms over multiple runs with the same rare earth chloride boat can be achieved with the same glass wool plug.

In summary, one of the features of the present invention is the uniform, continuous, high concentration rare earth vapor carried by an inert gas under oxygen-free conditions and using a rare earth boat that continuously provides a constant size of solid rare earth to be vaporized over a period of timeAn exposed surface. Moreover, because a ring heater surrounding the rare earth boat is used, a high vapor pressure can be formed to obtain a high concentration of rare earth vapor, which is then reacted with AlCl₃Such as a rare earth-producing intermediate, which is separated from the rare earth-producing process before mixing due to the separation effect of the flow regulator. If an intermediate exists in the rare earth generation process, high-concentration rare earth steam cannot be generated. Tuned separation is achieved by using a first inert quartz wool plug of predetermined porosity. The solid rare earth source forms high vapor pressure at the position of the rare earth boat due to the use of the ring heater, and rare earth vapor with high concentration is obtained. A second inert quartz wool plug of predetermined porosity acts as a second flow regulator so that the higher concentration (at least 2% or more) rare earth vapors formed are mixed with the intermediate before entering the reaction zone for mixing with other glass-forming components. In addition, it is important that the second intimate plug prevents oxygen from entering the rare earth component vapor formation zone. It is important to determine the thickness and porosity of the glass wool plug according to the corresponding transfer tube diameter, which must take into account that the pressure build up on the outer tube 220 is higher than that of the inner tube 200. The rare earth-bearing chloride vapor and aluminum chloride-bearing vapor must be able to readily permeate the glass wool plugs when considering the porosity of the glass wool, but the porosity is not so great as to permit the backflow of other oxygen-containing gases into the tubes 220 and 200. In fact, we have found that the outer surface 222A of the outer glass wool plug 222 forms a certain amount of oxides of glass forming substances, thereby preventing oxygen from penetrating the plug. The glass wool plugs 202 and 222 serve three functions. First, they are flow regulators of the rare earth and intermediate vapors forward, i.e., downstream. Second, they prevent reflux. Third, they trap oxides formed on the outer surface of the glass wool filter plug without allowing oxygen to back flow through the filter plug and contaminate the rare earth vapors that will enter the tube 5 near the upstream heated reaction zone and thoroughly mix and react with the glass-forming components.

Optical fibers made by HVP alone have very desirable rare earth chloride concentrations, as high as 4 wt%, as determined. See examples 1, 1A to 1C, 1D and 1F, examples 3, 3A, 3B to 3C and 3D. From a comparison of example 1 with example 3, it can be seen that the very high reaction temperatures at which the deposited glass layer immediately becomes consolidated glass, as in us 4,909,816, or the stepwise powder-on-powder sintering of the deposited particles, as in us 4,217,027, do not produce significant differences in achieving the high concentrations described.

Although a sufficiently high rare earth dopant concentration can be obtained using the HVP method alone to be suitable for most fiber lasers, higher concentrations can be obtained if used in combination with the aforementioned HLP method.

According to the HLP method, a predetermined concentration and structural distribution of the rare earth-containing dopant suitable for obtaining the desired optical properties can be further achieved by introducing the dopant in succession by combining soot deposition and solution doping. By such precisely controlled incremental introduction, an optical fiber preform containing a rare earth-containing dopant at a desired uniform concentration that cannot be achieved by either of the above-described methods alone can be obtained.

Soot deposition is generally carried out as follows: a stream of glass-forming precursor material vapour is first introduced into the cavity of hollow tube 5 and the precursor is then oxidised at a sufficiently high temperature and for a sufficiently long time to deposit at least one porous or particulate powder layer on the surface of the cavity. The composition of the vapor includes a first portion of the rare earth dopant. According to the invention, the dopant comprises a source of rare earth ions, i.e. ions of the lanthanide series of atomic numbers 57-71.

It has to be noted that, since the deposited layer is also solution doped, the temperature must be high enough for the well-known vapour phase oxidation reaction to take place, but not so high that the deposited layer sinters, which is important for the realisation of the invention. The end result should be a porous or particulate layer having a density of about 0.5 g/cc. At higher temperatures, the oxidic particles may sinter immediately after deposition, resulting in a glass layer which, because of its monolithic glass nature, cannot be impregnated any more with subsequent liquid doping solutions. At the end of the dust deposition, the tube cavity should be covered by an opaque powder frost layer by visual observation, and the tube cavity should be covered by a porous or granular powder layer by microscopic observation, wherein gaps, holes, cracks and/or capillaries are uniformly distributed.

The dopant solution is introduced into the interstices of the soot deposit under conditions suitable for absorption of the solution by at least one of the porous or particulate deposit. According to the invention, the dopant solution comprises a second portion of dopant. Like the first portion of dopant, this second portion of dopant also includes a source of rare earth ions. And the rare earth component can be further doped into the optical fiber perform by dipping the powder into the target powder layer, so that the final concentration of the rare earth in the optical fiber perform can be further improved. The first and second doping may also include introducing a refractive index adjusting or modifying component, such as a halide or oxide of Al, B, or P, in vapor or liquid form.

In a preferred embodiment, the powder layer is immersed in the desired solution for a relatively long time (e.g. several hours at room temperature). The impregnation process may be accelerated by evacuation and/or heating. Specifically, the vessel in which the soot-coated tube is located is pumped to a low pressure (either vacuum or heat or both) and then a dopant solution sufficient to submerge the soot-coated tube is added to the vessel. When the voids of the target powder layer are completely impregnated, the remaining solution is poured off. The dopant-impregnated soot layer is then dried in an atmosphere or inert atmosphere or vacuum at about 150-250 deg.c. The temperature is then raised to 750-850 deg.C under oxygen or oxygen-rich atmosphere to oxidize the rare earth dopant precursor (e.g., rare earth chloride). The solution dip operation may be repeated several times to increase the dopant concentration. Chlorine and SOCl can be introduced into the reaction atmosphere₄、CCl₄Or SF₆Gas to promote dehydration or water diversion.

After solution doping, the hollow tube is heated at a sufficiently high temperature for a sufficiently long time to sinter the powder layer deposited therein, and the tube is then sintered, the sintering and sintering may be carried out simultaneously or sequentially. In a preferred embodiment, the tube with the dopant soot layer is sintered to a consolidated preform above 2,000 ℃.

The resulting rare earth dopant doped in the optical fiber preform 10 includes the first and second portions of dopant. Since there may be other opportunities for introducing dopants and for losing part of the already doped material, the final concentration of dopants, although shown as two doping steps, is not necessarily exactly the sum of the two amounts, and may be slightly more or slightly less.

Although, as described herein, one skilled in the art can develop or utilize a variety of approaches to performing the HVP process and the HLP process, the VPD apparatus of FIG. 1 can be used to accomplish the vapor deposition step of either or both processes. The present invention contemplates the following implementation. In the first method, the HVP method uses a solid rare earth dopant whose exposed size during the vaporization process is constant. This can be exemplified in FIG. 1 using a rare earth chloride boat 32. In the second approach, the HVP process uses both a rare earth chelate vapor source 124 and a solid rare earth source 32 to achieve a higher rare earth component concentration or total rare earth component concentration in the glass forming mixture (note that the two rare earth sources may yield different rare earth components, such as co-doped Er and Yb). In the third method, the HVP method is combined with the HLP method in the first method, thereby increasing the concentration of the rare earth dopant in the deposited powder layer. In a fourth method, the HVP process of the second method is combined with the HLP process to achieve a maximum concentration of rare earth dopant in the deposited powder layer, and then the layer is dehydrated and sintered to form a monolithic glass layer with high rare earth concentration and high optical uniformity.

Referring now to fig. 7, a modification of the HVP process of the invention is shown. Like numerals in fig. 1 and 7 refer to like parts. The improvement in FIG. 7 is a conveyor system 20A in which there are a plurality of rare earth boats 32 and 32A adjacent to each other. Each rare earth boat is prepared as described above, one of which may be a halide of one rare earth and the other a halide of another rare earth, e.g., ErCl₃And YbCl₃. The present invention includes that the boats 32 and 32A are the same rare earth halide, or there may be multiple such rare earth boats, each having a respective ring heater 360 and 360A, with the respective heat being concentrated in each boat, thus controlling the vaporization of each rare earth source. Thus, depending on the vapor pressure of each rare earth component in the tube 200, the heaters 360 and 360A can be controlled separately to provide the desired ratio of rare earth vapors from the boats 32 and 32A, respectivelyThe gases, fed into the main glass-forming component vapor stream through plugs 202 and 222, mix in zone 5B.

Reference is now further made to fig. 8, which shows another modification of the present invention. Multiple quartz glass delivery system 800 has multiple delivery tubes 802, 804, and 806 that independently provide a series of gases that are generated, mixed, and eventually mixed with the glass-forming component vapors from delivery tube 808, as in the system shown in fig. 1. Thus, the invention includes the initial generation of rare earth vapors and other metal vapors (e.g., intermediates (e.g., AlCl)₃) And then mixed with oxygen and other oxides in the glass-forming constituent vapors in mixing zone 830 and then flowed downstream to react and deposit on the inner wall of tube 5. As in the embodiment of fig. 1, the innermost tube 802 may have a boat 818 loaded with a solid rare earth halide (e.g., rare earth chloride) and an inert gas (e.g., helium) enters the tube 802 through an inlet 803. The heater 822 forms a heating zone 826 for gasifying the rare earth into airborne form,the vapor enters tube 806 through gas permeable glass wool plug 812 and enters heating zones 828, 828 in another boat 820 which may contain another solid rare earth halide or intermediate whose vaporization rate is determined by the respective temperatures of heater 824 and heating zone 828. To achieve higher rare earth vapor concentrations, the rare earth is vaporized in region 826 before being mixed with another component vapor in region 828, and the gases produced in regions 826 and 828 are highly susceptible to contamination by oxygen or other oxides of glass-forming materials that may be present, but are protected by glass plugs 812, 814 and 816, particularly plugs 812 and 814. Prior to introducing the oxygen-containing glass-forming component vapor, it may be premixed in the chamber 829 of the third tube 806 with the other non-oxygen-containing components in the tubes 802 and 804 and then mixed with the glass-forming substance by passage through the glass wool plug 829 into the mixing zone 830. The above mixing may be, for example, rare earth vapor containing inert gas, additional intermediate or glass dopant. It should be noted that the present invention contemplates that delivery tubes 804 and 804 may be arranged adjacent to each other within the lumen of tube 816 such that the highest concentration of rare earth vapor may be provided from the respective solid rare earth by the respective inert gas and then within the lumen of tube 816The 829 zones within tube 806 mix. Thus, boats 818 and 820 may contain the same or different solid state rare earths, or alternatively, one boat may contain other solid state raw materials used in the glass forming process, such as glass dopants or intermediates. The final optical fiber preform 10 can be used as a starting material for producing various optical fiber products. However, due to the high rare earth concentrations achievable by the method of the present invention, the resulting optical fiber preform 10 is particularly suitable for use in the production of double clad fiber lasers. The glass core in a fiber laser, like other fiber products, is the portion that allows light to pass from one end of the fiber to the other. It can be either single-mode or multimode, but it must contain the active rare-earth ions necessary for the laser. To help retain the light propagating in the core, it is desirable to have an outer cladding surrounding the inner cladding and the core of the optical fiber. However, before drawing and wrapping such optical fibers, consideration must be given to pre-draw finishing of the optical fiber preform after sintering, such as "changing the liner".

Regarding such modifications, it is noted that optical fibers drawn from concentric circular preforms are inferior in optical performance, while optical fibers drawn from eccentric circular or rectangular or polygonal preforms are superior. However, the cost of creating these shapes is high and the match with some standard optical fibers on the market is poor. Recently, better optical performance has been achieved by making preforms with a substantially circular cross-section, then grinding slightly to introduce slight deviations in the otherwise perfectly circular cross-section, and drawing optical fibers from such preforms. Such small defects, which have proven to be very useful for enhancing the mode perturbation, are the introduction of simple flat surfaces or concavities in the cylindrical preform, which fortunately are very convenient to grind, requiring only a few passes over a suitably shaped grinding tool. Two very small flat surfaces are ground on opposite sides of an essentially circular cross-section, and an optical fiber having excellent light absorption properties is drawn from such a preform. See, for example, FIG. 2, which shows that the A-A and B-B portions can be removed by grinding to form the flats 13 and 15.

The slight defect of the preform after trimming acts as a mode disturbance. In this regard, it should be appreciated that as light propagates through a circular fiber by internal reflection, some of the light will be continuously internally reflected down the length of the fiber path, with the surfaces of the internal path repeatedly turning in the same geometric manner, possibly turning at all internal surfaces and not propagating completely into the center of the fiber. In a conventional double clad fiber laser, the central region is a core containing an active gain material for concentrating input light to emit laser light. If the input light is unable to propagate into the central region, it is not absorbed and the resulting laser intensity is reduced. The slight imperfections help to avoid the consequence of significantly changing the internal reflectivity of the fiber, in particular in relation to the effective angle of refraction within at least one internal surface, which is otherwise a constant internal circular shape, thereby disturbing the successive repetitive internal refraction patterns that may occur.

It is believed that the use of two planes, rather than one, three or more, is preferred. One of the disadvantages of a flat surface is that the resulting fiber is asymmetric and therefore not easily fusion spliced or connected to other fiber components. Three or more planes will produce more angles than two planes and internal scattering will be more, but more work is added, thus increasing cost.

In summary, it has now been found that planes 13 and 15 having a depth of about 5% to 10% of the diameter of the inner cladding 5 of the fiber are sufficient to obtain good mode-disturbing effects, although less desirable but still acceptable, if the depth is about 1-25%. In this regard, depths less than 1% have no significant effect, while the amount of work required to grind a depth of 25% or more is far from necessary to produce the desired mode disturbing effect, and also affects the matching with other fiber components and fibers.

While mechanical grinding is considered the simplest method of creating a slight defect in the optical fiber preform 20, it is contemplated that in other embodiments, the angular reflectivity of the optical fiber within at least one of the inner surfaces may be altered by chemical means. For example, by ion diffusion, material at the surface of the optical fiber is lost to some ionic salt bath, resulting in a change in the otherwise uniform surface optical properties.

Other ways will be apparent to those skilled in the art from the disclosures and suggestions relating to enhancing the mode scrambling. More information is available in published PCT application WO97/12429, 4/3/1997.

Moreover, although unique properties can be obtained by fabricating a preform by the method of the present invention and then performing the post-fabrication enhancement mode-disturbing process described above, its use can be generalized to other preform fabrication methods. For example, the processing of the enhanced mode perturbation may be used for preforms made by the so-called modified chemical vapor deposition Method (MCVD) (see U.S. Pat. No. 4,909,816(MacChesney et al)), or for preforms made by the so-called outside vapor oxidation method (OVPO) or outside vapor deposition method (OVD) (see U.S. Pat. No. 4,062,665 to Izawa et al).

After the hollow tube is collapsed into a solid collapsed preform without any internal voids, whether post-fabrication processing is employed or not, the optical fiber can be fabricated in a conventional manner by inserting one end of the preform into a furnace to heat the preform, as shown in FIG. 3. After the preform is heated, the preform may be drawn by a draw bar (bait rod) or other means in one or more steps into an optical fiber that retains the cross-sectional structure of the original preform.

Next, an outer cladding layer 40 is deposited over the drawn fiber, which can be done using a variety of conventional techniques known to those skilled in the art. However, in a preferred method, the drawn fiber is coated with a photopolymerizable composition, such as U.S. patent 5,534,558 to r.a. minss et al, 7/9 1996, which uses the apparatus shown in fig. 3B. The apparatus shown in fig. 3B has an oven 2 in which a glass preform is placed. Below oven 16 are two coating cups 4 and 6, each containing a photopolymerizable topcoat composition. Below the coating cup 6 is an ultraviolet lamp, such as a Fusion Research electrodeless ultraviolet mercury vapor lamp, and below the lamp 8 is a capstan 7. The device also has a reel 9. As capstan 7 pulls the optical fiber from oven 16 at a conventional rate of 0.5 inches per second, the pulled fiber 10 passes through coating cups 4 and 6. These cups have a conical base with a downward pointing cone tip, and the apex of each cone has a vertical hole through the base. The inner diameter of the hole is equal to the desired diameter of the optical fiber after the optical fiber is coated with the photopolymerizable composition, so that the hole serves to remove the excess photopolymerizable composition from the optical fiber. Then, the optical fiber coated with the uncured photopolymerizable composition is passed through the ultraviolet lamp 8, and the photopolymerizable composition solution is then cured, forming an adhered transparent clad on the optical fiber 10.

If desired, the coated fiber 10 may be passed through a coating cup and an ultraviolet lamp to apply a durable outer coating 50 to protect the generally softer outer polymer coating 40 from damage before it is wound onto wheel 9. After coating 50 is completed, the coated optical fiber 10 is wound around wheel 9.

Finally, when the continuous drawn fiber is split and used in a fiber amplifier or fiber laser or other fiber gain medium, it must be kept in mind that, in practical terms, the glass fiber length used in such lasers should not be too long nor too short. In particular, it is preferred to manufacture such lasers and/or amplifiers with short fibers but with sufficiently high rare earth dopant content such that substantially all of the incident pump light focused on one of the ends is absorbed after passing through one or at most two of the fibers. More specifically, if the device is used as a laser, almost all of the used incident pump light must be absorbed after passing through one or both of the fibers, but if the device is used as an amplifier, almost all of the used incident pump light must be absorbed after passing through one of the fibers.

The invention will be described in further detail below with reference to examples of several embodiments. These examples are intended only to illustrate the HVP and HLP processes of the invention and do not limit the scope of applicability of the invention, they are intended to illustrate the way in which the process of the invention can be used in practice in order to obtain preforms which can be modified in various ways to obtain optical fibres suitable for various specific applications. All parts, percentages, ratios, etc. are by weight unless otherwise specified.

Examples

Example 1

An optical fiber preform was prepared using the basic glass deposition composition and parameters in Table 1.

TABLE I

Formation of the cladding (1450 ℃ C. hot zone, 3 passes)		Core formation (1450 ℃ C. hot zone, 4 passes)
				Components	Flow rate	Components	Flow rate
SiCl4	500cc/20℃，1.1g/min	SiCl4	200cc/22℃，0.66g/min
						GeCl4	25cc/20℃，0.03g/min
POCl3	687cc/20℃，0.16g/min	POCl3	30cc/20℃，0.007g/min
				SF6	0.8cc/min	SF6	0.25cc/min
O2	1,000cc/min	O2	1,000cc/min
				He	1,000cc/min	He	1,000cc/min

^*Sintering at 1960 DEG C

More specifically, during the deposition process, a stream of glass-forming component vapor containing the above-described components is introduced into the chamber of quartz tube 5 through outer transport tube 240 of multiple concentric transport system 20 of fig. 1 and reacts to form a soot layer. As shown in the table, these soot layers, which will later form the inner cladding of the preform, are sintered at the end of the deposition, i.e. after the third pass.

During the core layer deposition, a helium vapor stream (flow rate about 300cc/min) was passed through aluminum chloride, which was introduced into column 122 of FIG. 1 and heated to 120-. The resulting helium stream carries AlCl₃The vapor is introduced into the chamber of the quartz tube 5 through the delivery tube 220 of the multiple concentric delivery system 20. At the same time, another helium stream (also at a flow rate of about 300cc/min) was passed through ytterbium chloride contained in the rare earth chloride boat 32 in the middle transfer tube 200 and heated to about 910-.

The resulting helium stream loaded with rare earth chloride vapor was further diluted with helium gas (again at a flow rate of about 300cc/min) and then introduced into the chamber of quartz tube 5. AlCl₃The vapor stream and the ytterbium chloride vapor stream are dispersed into a fine stream through the glass plugs 202 and 222 into the primary glass-forming component vapor stream. The powder layer was then dehydrated by contact with a stream of 50cc/min chlorine gas for 1-2 hours, followed by sintering.

The core composition of the final optical fiber preform was determined according to standard electron probe microanalysis. The data obtained in mol% are: 98.4% SiO₂，0.65％Al₂O₃，0.6％GeO₂，0.3％Yb₂O₃. Further measurements showed that the moisture content of the optical fiber preform was low. The measured attenuation is very low, with attenuation of only around 4dB/Km at laser wavelengths above 1 μm. The decay curve is shown in figure 4. Examples 1A to 1C

Three optical fiber preforms (examples 1A-1C) were prepared in the manner of example 1. However, ytterbium chloride in example 1A gasified at 930 deg.C, example 1B at 950 deg.C, and example 1C at 980 deg.C. It was observed that the core depositions for examples 1A-1C were all the same, but the ytterbium oxide content was increased from 1A to 1C, i.e., with increasing vaporization temperature. The ytterbium oxide concentration in example 1C exceeded 3 wt%. Example 1D

An optical fiber preform was prepared in the same manner as in example 1. However, in this embodiment, the core layer is formed not by 4 times but by 8 times. The resulting optical fiber preform has a core composition similar to that of the preform of example 1, but with a larger core diameter, and is therefore more suitable for drawing multimode optical fibers. Example 1E

An optical fiber preform was prepared in the same manner as in example 1. However, in this example, instead of ytterbium chloride, erbium chloride was heated at 910 ℃ to form a rare earth chloride vapor. Er of the obtained optical fiber preform₂O₃The concentration was about 2.5 wt%. No devitrification due to the formation of clusters was observed. Example 1F

A Yb: Er co-doped optical fiber preform was prepared in the same manner as in example 1. However, in addition to ytterbium chloride, another chloride boat was loaded with erbium chloride and placed next to the ytterbium chloride boat in the central transfer tube 200. Both rare earth chlorides were gasified at 998 ℃. The resulting optical fiber preform is homogeneous and has a relatively high Er₂O₃Concentration, and Yb of 4.0 wt%₂O₃And (4) concentration. Example 2

The optical fiber preform is prepared by combining soot layer deposition and solution doping techniques. The deposition of the soot layer was carried out as in example 1, but with the basic glass deposition components and parameters listed in table II.

TABLE II

Formation of the cladding (1450 ℃ C. hot zone, 3 passes)		Core formation (1450 ℃ C. hot zone, pass 2 times)
				Components	Flow rate	Components	Flow rate
SiCl4	500cc/20℃，1.1g/min	SiCl4	100cc，0.33g/min
						GeCl4	20cc，0.022g/min
POCl3	678cc/20℃，0.16g/min
				SF6	0.8cc/min
O2	1,000cc/min
				He	1,000cc/min

^*Sintering at 1960 DEG C

Unlike example 1, the soot layer is not sintered immediately after deposition, and the soot preform is placed in a tubular container and evacuated to 1-10 deg.f^-1And (5) Torr. The components are shown in Table IIIThe solution is drawn into the vessel under vacuum and the tube is covered with a layer of powder by immersion.

TABLE III

Doping solution
		Components	Measurement of
H2O	40cc
		Al(NO)3·9H2O	1.28g
Yb(NO)3·5H2O	6.14g
		Er(NO)3·5H2O	2.66

After about 2 hours, the doping solution was poured off and the soot-coated tube was dried at about 150 ℃ and 250 ℃. The above impregnation process was repeated.

And then dehydrating, calcining, sintering and sintering the powder layer covering tube to obtain the codoped optical fiber preform. By containing Cl₂(flow rate 50-80cc/min) and O₂(flow rate 1,000cc/min) the vapor was dehydrated at about 150 ℃ for about 30 minutes and then at about 750 ℃ and 800 ℃ for about 2 hours. At about 1960-₂And He (both flow rates are 1000cc/min) are passed through the lumen, whereupon sintering and sintering are completed in one pass.

The final codoped optical fiber preform core composition contained 2.5 wt% Yb as determined by standard electron probe microanalysis₂O₃，0.3wt％Er₂O₃And 97 mol% or more of silica. Example 3

An optical fiber preform was prepared using the basic glass deposition composition and deposition parameters set forth in Table IV.

TABLE IV

Formation of the cladding (1830 ℃ Hot zone, 4 passes)		Core formation (1800 + E
				Components	Flow rate	Components	Flow rate
SiCl4	500cc/20℃，1.1g/min	SiCl4	200cc/22℃，0.66g/min
						GeCl4	25cc/22℃，0.03g/min
POCl3	678cc/20℃，0.16g/min	POCl3	30cc/min/20℃， 0.007g/min
				SF6	0.8cc/min	SF6	0.25cc/min
O2	1,000cc/min	O2	1,000cc/min
				He	1,000cc/min	He	1,000cc/min

More specifically, during deposition, vapors of glass-forming constituents comprising the above-described constituents are introduced into the chamber of quartz tube 5 via outer delivery tube 240 of multiple concentric delivery system 20 of FIG. 1. However, unlike examples 1 and 2, the high temperatures used during each pass immediately produced a single glass deposit on the inner surface of the quartz tube 5.

During core deposition, a helium vapor stream (flow rate about 300cc/min) was passed through aluminum chloride, which was loaded into the 122 column of FIG. 1 and heated to 120-. The resulting helium stream carries AlCl₃The vapor is introduced into the chamber of the quartz tube 5 through the delivery tube 220 of the multiple concentric delivery system 20. At the same time, another helium stream (also at a flow rate of about 300cc/min) was passed through ytterbium chloride contained in the rare earth chloride boat 32 in the middle transfer tube 200 and heated to about 910-.

The resulting helium stream loaded with rare earth chloride vapor was further diluted with helium gas (flow rate about 700cc/min) and then introduced into the chamber of quartz tube 5. AlCl₃With ytterbium chloride vapor through a glass wool filter plug 202 and 222 into a fine stream into the vapor of the basic glass-forming component and then react at an extremely high temperature, i.e., 1800-. The resulting tube is then collapsed into an optical fiber preform.

The core composition of the final optical fiber preform was determined according to standard electron probe microanalysis. The data in mol% are identical to those of example 1: 98.4% SiO₂，0.65％Al₂O₃，0.6％GeO₂，0.3％Yb₂O₃. Example 3A

A preform was prepared in the manner of example 3. But instead of forming amorphous glass immediately during the core layer deposition process at extremely high temperatures, several layers of powder are deposited at a lower temperature of about 1650 c. Then using 50cc/min Cl₂The soot layer was treated with gas for 1-2 hours to enhance water removal and then sintered into a uniform glass layer at about 1980-2000 ℃. The hollow tube is sintered into a solid cylindrical optical fiber preform. The composition of the resulting preform was similar to that of the preform of example 3. The attenuation is very low, about 4dB/Km for laser wavelengths above 1 μm. Examples 3B to 3D

3 optical fiber preforms (i.e., examples 3B-3D) were prepared using the method of example 3. However, the ytterbium chloride of example 3B gasified at about 930 deg.C, example 3C gasified at about 950 deg.C, and example 3D gasified at about 980 deg.C.

Analysis revealed that the core compositions of examples 3B-3D were essentially the same, but the ytterbium oxide concentration increased from 3B to 3D, i.e., with increasing temperatures used for vaporization. Example 3C the ytterbium oxide concentration in the optical fiber preform exceeded 3 wt%. Example 4

An optical fiber preform of the present invention is prepared using cyclopentadienyl neodymium as a dopant precursor. The base glass deposition compositions and parameters are listed in table V.

TABLE V

Formation of the cladding (1830 ℃ Hot zone, 4 passes)		Core formation (1790 + 1820 ℃ C. hot zone, 4 passes)
				Components	Flow rate	Components	Flow rate
SiCl4	500cc/22℃，1.1g/min	SiCl4	200cc/22℃，0.66g/min
						GeCl4	25cc/22℃，0.03g/min
POCl3	678cc/22℃，0.16g/min	POCl3	40cc/min/20℃
				SF6	0.8cc/min
O2	1,000cc/min	O2	1,000cc/min
				He	1,000cc/min	He	1,000cc/min

Specifically, a helium stream (flow rate about 300cc/min) was passed through aluminum chloride, which had been previously packed in the 122 column of the VPD apparatus of FIG. 1 and heated to about 120-. The resulting helium stream carries AlCl₃The vapor is introduced into the chamber of the quartz tube 5 through the transfer tube 220. Simultaneously, another helium stream (flow rate about 300cc/min) was passed through the compound cyclopentadienylneodymium (ND-CP)₃) The ND-CP₃Previously loaded in a 124 column and heated to about 230 ℃. The resulting helium stream is loaded with ND-CP₃The vapor is introduced into the chamber of the quartz tube 5 through the transfer tube 200. (there is no chloride boat 32 in the central transfer tube 200). AlCl₃And ND-CP₃The vapor stream is dispersed into a fine stream through the glass wool plugs 202 and 222, respectively, into the base glass vapor, which has a composition as shown in the above table, which passes throughThe transfer tube 240 is introduced into the quartz tube 5. The helium gas streams carrying the various vapors reach the quartz tube 5 where they mix and react at extremely high temperatures (i.e., about 1800-. The tube is collapsed to obtain the final optical fiber preform.

The core deposition layer of the resulting optical fiber preform was determined according to standard electron probe microanalysis. The concentration of neodymium oxide in the preform was 1 wt% and the silica concentration exceeded 97%. The core of a double clad fiber laser prepared from this preform according to the methods of U.S. Pat. Nos. 4,815,079(Snitzer et al) and 5,534,558(Minus) exhibits very low attenuation, about 10dB/Km for the wavelength of 1000-1200nm laser. The efficiency slope of the double clad fiber laser is over 50%. Example 5

An optical fiber preform was prepared in the same manner as in example 2. But higher temperatures are used to deposit the cladding layer, which results in faster formation of the amorphous glass deposit. To accommodate such high temperatures, the basic glass deposition composition and parameters need to be modified as shown in Table VI.

TABLE VI

Formation of the cladding (1830 ℃ Hot zone, 4 passes)		Core formation (1450 ℃ C. hot zone, pass 2 times)
				Components	Speed of rotation	Components	Speed of rotation
SiCl4	500cc/22℃，1.1g/min	SiCl4	100cc/22℃，0.33g/min
						GeCl4	20cc/22℃，0.22g/min
POCl3	678cc/22℃，0.16g/min
				SF6	0.8cc/min
O2	1,000cc/min
				He	1,000cc/min

Otherwise, the process was the same as example 2, including a solution doping step after the core soot layer was deposited. In any event, the core composition of the resulting optical fiber preform was similar to that of the preform of example 2, i.e., 2.5 wt% Yb, as determined by standard electron probe microanalysis₂O₃And 0.4 wt% Er₂O₃The silica concentration is over 97%.

In view of the above, the present invention does provide a very efficient method of forming an optical fiber preform and corresponding apparatus, thereby achieving the objects of the present invention. It is also obvious and conceivable that modifications can be made to the embodiments described above, without departing from the scope of the present invention. While various embodiments have been described above, these are by way of example only, and not by way of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit of the invention. Accordingly, the present invention is not limited to the scope of the above embodiments, but should be defined by the claims and their equivalents.

Claims (80)

1. An apparatus for delivering components in the manufacture of a glass fiber preform comprising:

a main glass tube;

a plurality of smaller tubes for insertion into one end of said main glass tube, each of said tubes having an input end and an output end, the output ends being the ends which extend most into the main glass tube;

the output end of the innermost one of the plurality of small tubes has a first plug which allows forward flow into the next tube but prevents reverse flow into the first tube;

a first tube of the plurality of small tubes provides a first chamber having a solid first glass component therein;

a second tube of the plurality of tubes providing a second chamber for the second glass component to mix with the vapor of the first component after passing through the plug of the first tube, the second tube having a second plug at its output end that allows forward flow into the next tube but prevents reverse flow into the second tube;

the mixture of components is mixed with other glass-forming substances within the main glass tube after passing through the multi-tube filter plug, such that the other glass-forming substances do not enter the first and second chambers through the first and second filter plugs.

2. The apparatus of claim 1 wherein at least one of said chambers contains a boat containing a solid glass composition.

3. The apparatus of claim 2 wherein a stream of inert gas is introduced at the input end of the tube in which the boat is located.

4. The apparatus of claim 3, further comprising a vapor of said glass component entering at the input end of said boat tube.

5. The apparatus of claim 4, the solid and gaseous glass components comprising rare earth halides.

6. The apparatus of claim 5, said rare earth halide being a chloride of Nd, Yb, Er, Tm, Ho, or Sm.

7. The apparatus of claim 2 wherein a heater is provided around the boat-located portion of the boat-located tube.

8. The apparatus of claim 2 wherein said boat has an exposed two-dimensional surface of said solid glass composition.

9. The apparatus of claim 1, wherein the first glass component is a rare earth halide.

10. The apparatus of claim 9 wherein the rare earth halide is vaporized and carried by an inert gas entering at the input end of the tube.

11. The apparatus of claim 10 wherein said inert gas is helium.

12. The apparatus of claim 9 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

13. The apparatus of claim 1, wherein at least one of said lumens is surrounded by a heater, wherein an elevated temperature is formed within the lumen.

14. The apparatus of claim 13, wherein the heater is a ring-shaped resistive heater or a radio frequency heater.

15. The apparatus of claim 13 wherein the boat is located within the chamber in which the solid components are located.

16. The apparatus of claim 15 wherein said boat has an exposed two-dimensional surface of said solid glass component and said heater develops an elevated temperature within said chamber to vaporize said solid glass component.

17. The apparatus of claim 16 wherein the glass constituent is a rare earth halide.

18. The apparatus of claim 17 wherein the rare earth halide is vaporized and carried by an inert gas entering at the input end of the tube.

19. The apparatus of claim 18 wherein said inert gas is helium.

20. The apparatus of claim 17 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

21. The apparatus of claim 1 wherein at least two of said lumens each have at least one boat containing a solid glass-forming component.

22. The apparatus of claim 21 wherein at least one of the solid components is a rare earth halide.

23. The apparatus of claim 22 wherein the rare earth halide is vaporized and carried by an inert gas entering at the input end of the tube.

24. The apparatus of claim 23 wherein said inert gas is helium.

25. The apparatus of claim 24 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

26. The apparatus of claim 21 wherein said lumen is surrounded by a heater to create an elevated temperature within the lumen.

27. The apparatus of claim 21 wherein at least one of the solid components is a glass intermediate.

28. The apparatus of claim 27, the intermediate is a halide vapor of one or more of Al, Ga, In, As, and/or Sb.

29. The apparatus of claim 1, wherein at least some of the small tubes are concentrically arranged within the main glass tube.

30. The apparatus of claim 1, at least some of the tubules being in side-by-side relationship within the main glass tube.

31. The apparatus of claim 1, at least one of the small tubes being for introducing a plurality of glass-forming components into the main glass tube.

32. The apparatus of claim 1 having at least two boats within at least one of said lumens, said boats each containing a solid glass-forming component.

33. The apparatus of claim 32 wherein at least one of said boats contains a solid rare earth halide composition.

34. The apparatus of claim 33 wherein the rare earth halide is vaporized and carried by an inert gas entering at the input end of the tube.

35. The apparatus of claim 34 wherein said inert gas is helium.

36. The apparatus of claim 35 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

37. The apparatus of claim 32 wherein the chamber in which the boat is located is surrounded by at least one heater which creates an elevated temperature within said chamber.

38. A solid state glass component delivery system comprising:

a tube;

an inert gas stream provided through the tube;

a boat positioned at the output end of said tube, said boat containing a solid glass-forming composition and providing an exposed two-dimensional surface of predetermined length and width of said solid composition;

a heater for vaporizing said components into a vapor carried by the inert gas;

the components are gasified at a given temperature at a gasification speed determined by the two-dimensional surfaces exposed outside the components in the boat;

a plug secured to the output end of the tube, the plug allowing the passage of component vapors therethrough but preventing backflow into the tube such that the components vaporize without any other glass-forming components being present on the outside of the plug.

39. The solid state glass constituent delivery apparatus of claim 38 wherein the constituent comprises a rare earth halide.

40. The apparatus of claim 39 wherein said rare earth halide is vaporized and carried by an inert gas entering at the input end of the tube in which it is located.

41. The apparatus of claim 40 wherein said inert gas is helium.

42. The apparatus of claim 39 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

43. The solid state glass constituent delivery apparatus of claim 39 wherein the heater is a ring heater surrounding the periphery of the tube in the region of the boat.

44. The solid state glass constituent delivery apparatus of claim 43 wherein the heater is a resistive heater or a radio frequency heater.

45. The solid state glass constituent delivery apparatus of claim 38 wherein the plug is of an inert gas permeable material.

46. The solid state glass constituent delivery apparatus of claim 45 wherein said plug is formed of a porous frit.

47. The solid state glass constituent delivery apparatus of claim 38 wherein the delivery system has a plurality of small tubes inserted into one end of the main glass tube, each tube having an input end for inflow of an inert gas carrier or glass forming constituent vapor and an output end which is the end furthest into the main glass tube.

48. The solid state glass constituent delivery apparatus of claim 47 wherein at least some of the tubes are in concentric relation within the main glass tube.

49. The solid state glass constituent delivery apparatus of claim 47 wherein at least some of the tubes are in side-by-side relationship with the main glass tube.

50. The solid state glass constituent delivery apparatus of claim 47 wherein at least one of the tubes introduces a plurality of glass forming constituents into the main glass tube.

51. The solid state glass constituent delivery apparatus of claim 47 wherein at least one of the tubes has a chamber adjacent the output end thereof, the chamber having a boat containing the solid state glass forming constituent.

52. The solid state glass constituent delivery apparatus of claim 51 wherein a heater surrounds the boat in each tube.

53. The solid state glass constituent delivery apparatus of claim 38 wherein the tube material comprises quartz glass.

54. The solid state glass constituent delivery apparatus according to claim 38 wherein said tube is a quartz glass tube.

55. A method of fabricating an optical fiber preform, comprising:

(a) providing a solid glass-forming composition having an exposed two-dimensional surface of a predetermined length and width;

(b) subjecting the component to a sufficiently high temperature in an oxygen-free environment such that the component vaporizes from the exposed two-dimensional surface forming a vapor of the component;

(c) carrying the component vapors to a reaction zone within a hollow tube with a flow of inert gas;

(d) the glass-forming substance vapor is introduced into the reaction zone concurrently with the introduction of the component vapor and is left at an elevated temperature.

56. The method of claim 55, further comprising increasing the temperature of the reaction zone to form a component vapor and a vapor of the glass-forming substance to form at least one layer of the substance on the surface of the lumen, the layer of the substance being either a monolithic glass layer or a porous or particulate frit layer.

57. The method of claim 56 wherein the glass layer formed is a porous or granular soot layer and further comprising the step of solution doping the soot layer such that the dopant solution is absorbed into the soot layer deposition layer.

58. The method of claim 57 wherein the dopant solution contains a rare earth dopant.

59. The method of claim 56, further comprising a step of collapsing the hollow tube.

60. The method of claim 55 wherein said inert gas is helium.

61. The method of claim 55 wherein the glass component comprises a rare earth halide.

62. The apparatus of claim 61 wherein said rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

63. The apparatus of claim 61 wherein said rare earth halide is a mixture of one or more of the chlorides of Nd, Yb, Er, Tm, Ho or Sm.

64. A method of making an optical fiber comprising:

(a) providing an optical fiber preform made by the method of claim 57, said optical fiber having a refractive index n₂Has a refractive index n in the multimode cladding₁A substantially single mode core of (a);

(b) depositing a layer with refractive index n on the optical fiber preform₃The multimode cladding satisfies n₁＞n₂＞n₃；

(c) Whereby the optical fiber preform is drawn into an optical fiber.

65. The method of claim 64 wherein the further cladding layer is deposited from a UV-curable composition comprising a copolymer having pendant photoinitiating groups, repeating units derived from a photoinitiating monomer having both a photoinitiating group and an ethylenically unsaturated group, and repeating units derived from a fluorine substituted monomer having an ethylenically unsaturated group and a fluorine substituted diacrylate (diacrylate).

66. A multi-tube delivery system for introducing glass component vapors into a reaction zone, the multiple concentric delivery system comprising:

a plurality of concentrically arranged vapor transport tubes, the central vapor transport tube having an upstream end and a downstream end;

rare earth halides are contained within the central vapor delivery tube for vaporization of the rare earth halides into vapors at a sufficiently high temperature;

an inert gas stream entering from the upstream end of said central vapor delivery tube and exiting from the downstream end;

a glass forming substance vapor introduced into the reaction zone through a vapor delivery tube other than the central vapor delivery tube;

the downstream end of the central vapor delivery tube is closed with a plug that allows the rare earth halide vapor carried by the inert gas within the central delivery tube to permeate therethrough, but prevents the glass former vapor from entering the central vapor delivery tube.

67. A multi-tube delivery system according to claim 66, wherein the rare earth halide compound is contained in a containment vessel within the central vapor delivery tube, the vessel having an exposed two-dimensional surface from which the rare earth halide compound is vaporized.

68. A multi-tube delivery system according to claim 66, wherein the rare earth halide is a chloride of Nd, Yb, Er, Tm, Ho or Sm.

69. A method of fabricating an optical fiber preform having a core doped with a predetermined amount of a dopant material having a specific refractive index, said method comprising the steps of:

(a) introducing a vapor of a glass-forming species precursor into a hollow tube, the vapor comprising a first portion of a rare earth-containing dopant;

(b) oxidizing the precursor material at a temperature and for a time sufficient to deposit at least one layer of soot on the surface of the lumen;

(c) introducing a dopant solution into the lumen having the soot layer deposited thereon, the dopant solution containing a second portion of the dopant, the second portion of the dopant containing a rare earth compound; and

(d) heating the sintered soot layer and then collapsing the tube, either simultaneously or sequentially, to produce an optical fiber preform containing a predetermined amount of dopant, the predetermined amount including a first portion and a second portion of the rare earth compound.

70. The method of claim 69, wherein at least the dopant in one of steps (a) or (c) comprises a refractive index altering component.

71. The method of claim 70 wherein the refractive index altering component is one or more of halides or oxides of Al, B or P.

72. The method of claim 69 wherein said vapor is heated at least one rare earth compound with SiCl₄And then contacting the heated mixture with a carrier gas.

73. The method of claim 72, wherein the carrier gas is helium.

74. The method of claim 69 wherein the deposition of the powder layer is carried out at 1400-1650 ℃.

75. The method of claim 69 wherein the sintering is carried out at 1960-.

76. A method for obtaining a high concentration of rare earth dopant in a soot layer deposited within the cavity of a glass tube, said glass tube then being sintered into a glass fiber preform, the steps of the method comprising:

providing a rare earth dopant vapor independent of other glass-forming precursor components;

mixing rare earth dopant vapor with glass-forming precursor component vapor;

forming a powder layer on the inner surface of the glass tube cavity;

allowing the powder layer to absorb the liquid rare earth dopant;

sintering the absorbed powder layer to convert the powder layer into a single glass layer; and

the glass tube is collapsed into a preform.

77. The method of claim 76, further comprising the step of premixing the rare earth dopant vapor or liquid with the refractive index altering component prior to the mixing and absorbing steps.

78. The method of claim 76 wherein the rare earth dopant is a compound comprising one or more of Nd, Yb, Er, Tm, Ho, or Sm.

79. The method of claim 76, further comprising a step of dehydrating the powder layer prior to sintering.

80. The method of claim 79 wherein said dehydrating step comprises allowing Cl₂、SOCl₂、CCl₂Or SF₆Passing over the powder layer.

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