Method to prevent cracks in optical fiber preforms

Abstract

The present disclosure provides optical fiber preforms formed from core canes having large core-clad ratio, intermediate core-cladding assemblies, and methods for making the preforms and core cladding assemblies. The preforms are made with capped core canes. The capping material has a coefficient of thermal expansion less than the coefficient of thermal expansion of the core cane and more closely matched to or lower than the coefficient of thermal expansion of the surrounding cladding monolith in a cane-in-soot process. Presence of the cap reduces stresses that arise from differential thermal expansion of the core cane and cladding materials and leads to preforms having low defect concentration and low probability of failure during subsequent thermal processing steps.

Claims

1. A core-cladding assembly comprising: a porous soot cladding monolith, said porous soot cladding monolith including a first porous cladding glass layer surrounding an internal cavity, said porous soot cladding monolith comprising a first material having a first coefficient of thermal expansion; a first consolidated glass body having a portion positioned in said internal cavity, said first consolidated glass body comprising a second material having a second coefficient of thermal expansion, said second coefficient of thermal expansion differing from said first coefficient of thermal expansion, said first consolidated glass body having a centerline aligned along an axial direction of said internal cavity and an end surface, said end surface crossing said centerline; and a second glass body having a portion positioned in said internal cavity in direct contact with said end surface of said first consolidated glass body, said second glass body comprising a third material having a third coefficient of thermal expansion, said third coefficient of thermal expansion differing from said second coefficient of thermal expansion.

2. The core-cladding assembly of claim 1, wherein said first material comprises silica.

3. The core-cladding assembly of claim 1, wherein said second material comprises silica containing a dopant.

4. The core-cladding assembly of claim 3, wherein said first consolidated glass body has a core-clad ratio of at least 0.70.

5. The core-cladding assembly of claim 1, wherein said third material comprises silica.

6. The core-cladding assembly of claim 1, wherein said second coefficient of thermal expansion is greater than said first coefficient of thermal expansion.

7. The core-cladding assembly of claim 1, wherein said second coefficient of thermal expansion is greater than said third coefficient of thermal expansion.

8. The core-cladding assembly of claim 1, further comprising a third glass body having a portion positioned in said internal cavity, said third glass body comprising a fourth material having a fourth coefficient of thermal expansion, said fourth coefficient of thermal expansion differing from said second coefficient of thermal expansion.

9. A product formed by consolidating the core-cladding assembly of claim 1.

10. The core-cladding assembly of claim 3, wherein said dopant comprises Ge.

11. The core-cladding assembly of claim 3, wherein said first consolidated glass body has a core-clad ratio of at least 0.90.

12. The core-cladding assembly of claim 1, wherein said second material has a higher refractive index than said first material.

13. The core-cladding assembly of claim 1, wherein said third material has a lower refractive index than said second material.

14. The core-cladding assembly of claim 1, wherein said first consolidated glass body is wholly contained within said internal cavity.

15. The core-cladding assembly of claim 14, wherein said second glass body is wholly contained within said internal cavity.

16. The core-cladding assembly of claim 14, further comprising a third glass body having a portion positioned in said internal cavity, said third glass body comprising a fourth material having a fourth coefficient of thermal expansion, said fourth coefficient of thermal expansion differing from said second coefficient of thermal expansion.

17. The core-cladding assembly of claim 16, wherein said first consolidated glass body is in direct contact with said second glass body and said third glass body within said internal cavity.

18. The core-cladding assembly of claim 16, wherein said second glass body has a thickness in the range from 1.0 mm to 75 mm and said third glass body has a thickness in the range from 1.0 mm to 75 mm.

19. The core-cladding assembly of claim 1, wherein said second glass body has a thickness in the range from 1.0 mm to 75 mm.

20. The core-cladding assembly of claim 7, wherein said third coefficient of thermal expansion is greater than or equal to said first coefficient of thermal expansion.

21. The core-cladding assembly of claim 1, wherein said first consolidated glass body and said second glass body are stacked in an axial direction in said internal cavity.

22. The core-cladding assembly of claim 1, wherein said first consolidated glass body has a cylindrical shape.

23. The core-cladding assembly of claim 22, wherein said second glass body has a cylindrical shape.

24. The core-cladding assembly of claim 1, wherein the second glass body covers the end surface of the first consolidated glass body.

25. The core-cladding assembly of claim 8, wherein said third glass body directly contacts a second end surface of said first consolidated glass body.

26. The core-cladding assembly of claim 1, wherein said end surface faces an entrance of said internal cavity.

27. The core-cladding assembly of claim 1, wherein said end surface is flat.

28. The core-cladding assembly of claim 1, wherein said core-cladding assembly includes a gap between an outer surface of said first consolidated glass body and an inner surface of said porous soot cladding monolith.

29. The core-cladding assembly of claim 1, wherein said first consolidated glass body and said second glass body directly contact said first porous cladding glass layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the written description, it is believed that the specification will be better understood from the following written description when taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1A is a diagram illustrating an up-doped overclad profile for making a low bend loss optical fiber.

(3) FIG. 1B is a diagram illustrating an up-doped overclad profile with trench for making a low bend loss optical fiber.

(4) FIG. 2A is an exemplary relative refractive index profile of the optical fiber illustrated in FIG. 2A, the core extending from r.sub.0 to r.sub.1 and having an index Δ.sub.1, the inner cladding extending from r.sub.1 to r.sub.2 and having an index Δ.sub.2, and the over cladding extending from r.sub.2 to r.sub.3 and having an index Δ.sub.3.

(5) FIG. 2B is an exemplary relative refractive index profile of the optical fiber illustrated in FIG. 2B, the core extending from r.sub.0 to r.sub.1 and having an index Δ.sub.1, the inner cladding extending from r.sub.1 to r.sub.2 and having an index Δ.sub.2, a trench of a low refractive index material extending from r.sub.2 to r.sub.4, and having a refractive index of Δ.sub.4 and the over cladding extending from r.sub.4 to r.sub.3 and an index Δ.sub.3.

(6) FIG. 3 is a frontal view of a core soot preform formed using a method according to embodiments of the present invention.

(7) FIG. 4 depicts deposition of a soot layer on a bait rod.

(8) FIG. 5 depicts deposition of a second soot layer on the soot layer shown in FIG. 4.

(9) FIG. 6 depicts deposition of a third soot layer on the soot layer shown in FIG. 5.

(10) FIG. 7 depicts processing of a core-cladding assembly that includes a consolidated core cane inserted into an internal cavity of a three-layer soot cladding monolith.

(11) FIG. 8 depicts a fiber preform formed from the core-cladding assembly shown in FIG. 7.

(12) FIGS. 9(a)-(c) show a fiber preform prepared by a cane-in-soot process.

(13) FIG. 10 shows the thermal expansion coefficient of Ge-doped silica, undoped silica, and F-doped silica glasses.

(14) FIG. 11 shows the coefficient of thermal expansion of a preform having a Ge-doped silica core and an undoped silica cladding.

(15) FIG. 12 depicts the radial tensile stress distribution in the vicinity of the end surface of a core cane in a consolidated fiber preform made by the cane-in-soot process.

(16) FIG. 13 shows the dependence of radial tensile stress on corner radius.

(17) FIG. 14 is an image of a core cane exhibiting a defect.

(18) FIG. 15 depicts a capped core cane.

(19) FIG. 16 depicts one method of forming a capped core cane.

(20) FIG. 17 depicts formation of a core-cladding assembly by insertion of a prefabricated capped core cane into an internal cavity of a soot cladding monolith.

(21) FIG. 18 depicts an alternative embodiment for forming a core-cladding assembly having a capped core cane and a soot cladding monolith.

(22) FIG. 19 depicts consolidation of a core-cladding assembly having a capped core cane within an internal cavity of a soot cladding monolith.

(23) FIG. 20 shows the dependence of radial tensile stress at the corner region of a consolidated preform as a function of the height (thickness) of an SiO.sub.2 cap present at the end surface of a GeO.sub.2-doped silica core cane.

(24) FIG. 21 shows an image of a preform that includes a capped core cane.

(25) FIG. 22 depicts the radial tensile stress distribution in a preform made with a capped core cane.

(26) The embodiments set forth in the drawings are illustrative in nature and not intended to be limiting of the scope of the detailed description or claims. Whenever possible, the same reference numeral will be used throughout the drawings to refer to the same or like feature.

DETAILED DESCRIPTION

(27) Reference will now be made in detail to illustrative embodiments of the present description.

(28) As used herein, the term “soot” means SiO.sub.2 or doped-SiO.sub.2 particles; the individual soot particles generally have a size of 0.01 to 10 micrometers in diameter. “Soot preform” means an article made of soot particles that has an open porosity. The terms “porous soot” and “porous glass” are used interchangeably herein. The term “core portion” or “core cane” refers to consolidated glass and may comprise a silica or doped silica glass. The term “consolidated glass” refers to glass in a closed pore state. In some embodiments, the glass is void-free. The term “soot-to-glass transformation” refers to the process of going from a porous glass state to a closed porosity state. As described more fully hereinbelow, the soot-to-glass transformation process may include a dehydration step, a doping step, and a sintering step. In some embodiments, the glass becomes void-free in the soot-to-glass transformation process. The term optical fiber preform (or consolidated preform, sintered preform or blank) refers to a glass article from which an optical fiber can be drawn.

(29) As used herein, “up-dopant” refers to a dopant that raises the refractive index of a silica (“SiO.sub.2”) glass relative to that of pure undoped SiO.sub.2; that is, the up-dopant causes the glass refractive index to increase. The term “down-dopant” refers to a dopant that lowers the refractive index of a silica glass relative to that of pure undoped SiO.sub.2 glass. Examples, without limitation, of up-dopants include Ge, Al, P, Ti, Cl, and Br. Examples, without limitation, of down-dopants include non-periodic voids, fluorine and boron. Dopant concentrations in the preform and/or fiber are expressed herein on the basis of weight (e.g., ppm by weight, ppm (by weight), percent by weight, wt %).

(30) “Relative refractive index,” as used herein, is defined in Eq. 1 as:

(31) $\begin{array}{cc}{\Delta }_i\left(r_i\right)\%=100\frac{\left(n_i^2-n_{\mathrm{ref}}^2\right)}{2n_i^2}& \mathrm{Eq}.1\end{array}$
where n.sub.i is the refractive index at radius unless otherwise specified, and n.sub.ref is the refractive index of pure silica glass, unless otherwise specified.

(32) As used herein, “dehydration atmosphere” or “drying atmosphere” refers to a gas atmosphere that contains a “drying gas”; and “drying gas”, as used herein, refers to a gas or gas mixture including a desired and suitable drying agent. A drying agent is a compound for drying that acts by removing water and/or hydroxyl ions present in a soot preform. Exemplary drying agents without limitation include: CCl.sub.4 Cl.sub.2, Br.sub.2, SOCl.sub.2, CO, and SiCl.sub.4. For example and without limitation, a “drying gas” may be He, a mixture of He and N.sub.2, and one of Cl.sub.2, CCl.sub.4, SiCl.sub.4, and/or other drying agents. In some embodiments the drying gas includes a drying agent selected from the group consisting of Cl.sub.2 and SiCl.sub.4. Concentrations of components in the gas phase are expressed herein on the basis of volume (e.g., ppm by volume, ppm (by volume), percent by volume, vol %).

(33) As used herein, “consolidation” or “soot-to-glass consolidation” refers to heating a soot preform to temperatures of at least about 800° C. to perform various process steps such as drying, doping and sintering. In one embodiment, consolidation occurs over a temperature range of 800° C. to 1550° C. The stage of consolidation at which the soot preform is heated at temperatures below 1050° C. may be referred to herein as the pre-heat step or pre-heating step of consolidation. The pre-heating step may occur at a temperature in the range from 800° C. to 1050° C. In one embodiment, drying and doping are completed in the pre-heating step of consolidation. The stage of consolidation at which the soot preform is heated to a temperature between 1050° C. and 1300° C. may be referred to herein as the intermediate heat step or intermediate heating step of consolidation. The pre-heating and intermediate heating steps may be combined in a single step or may be performed sequentially. The stage of consolidation at which the soot preform is heated to a temperature of at least 1300° C. may be referred to herein as the sinter step or sintering step of consolidation. The sintering step may occur at a temperature in the range from 1300° C. to 1550° C., or at a temperature above 1550° C. It is expected that densification of the soot to form glass (e.g. to form vitreous or fused silica, or a doped fused silica) occurs primarily in the sintering step (but may also occur at the intermediate heating step and/or the pre-heating step). Doping the preform with chlorine using chlorine gas or a chlorine-containing dopant precursor may occur during one or more of the pre-heating step, intermediate heating step, and sintering step.

(34) As used herein, “cladding” refers to a glass layer or a plurality of glass layers surrounding an optical fiber core and extending from the outer edge of the core to the outer edge of the optical fiber; and the term “overclad” or “outer clad” and similar terms refer to the last cladding layer on the optical fiber (the outermost, most radially distant cladding layer).

(35) The core-clad ratio is defined herein as the ratio of the radius of the high index region of the core cane to the outer radius of the core cane.

(36) As used herein, contact refers to direct contact or indirect contact. Direct contact refers to contact in the absence of an intervening material and indirect contact refers to contact through one or more intervening materials. Elements in direct contact touch each other. Elements in indirect contact do not touch each other, but do touch an intervening material. Elements in contact may be rigidly or non-rigidly joined.

(37) The present disclosure is described more fully below with reference to the accompanying drawings. However, this disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

(38) Optical fibers generally consist of a core fiber for transmission of light and one or more cladding layers whose purpose is to keep the transmitted light within the core and reduce the transmission loss over distance. An exemplary step index optical fiber is shown in FIG. 1A. The optical fiber 11a includes a core 10, an inner cladding 12a and an outer cladding 12b. A relative refractive index profile for the optical fiber 11a of FIG. 1A is shown in FIG. 2A. The core 10 extends radially from a core center r.sub.0 to a core outer edge r.sub.1 and has an index Δ.sub.1. The inner cladding 12a extends radially from the core outer edge r.sub.1 to the radial distance r.sub.2 and has an index Δ.sub.2. The outer cladding 12b extends radially from r.sub.2 to the outer edge of fiber at r.sub.3 and has an index Δ.sub.3.

(39) It will be appreciated from the description herein that optical fibers of alternative suitable constructions can be formed in accordance with the present invention. In some embodiments, there is a trench region designated 12t between the inner and outer cladding regions. A trench region reduces the sensitivity of the optical fiber to bending loss. An example of such a profile is shown as optical fiber 11b in FIG. 1B, which has the relative refractive index profile shown in FIG. 2B. In FIG. 2B, the core 10 extends radially from a core center r.sub.0 to a core outer edge r.sub.1 and has an index Δ.sub.1. The inner cladding 12a extends radially from the core outer edge r.sub.1 to the radial distance r.sub.2 and has an index Δ.sub.2. The trench region 12t extends radially from r.sub.2 to r.sub.4 and has an index Δ.sub.4. The outer clad region 12b extends radially from r.sub.4 to the outer edge of fiber at r.sub.3 and has an index Δ.sub.3. The index Δ.sub.3 of the outer clad region is higher than the index in the inner cladding region Δ.sub.4 as a result of updoping of the outer cladding region. The index Δ.sub.4 of trench region 12t is achieved by doping the region with a down-dopant.

(40) The present description provides optical fiber preforms made from core canes having a large core-clad ratio, intermediate core-cladding assemblies used to make the optical fiber preforms, a process for making the preforms, and optical fibers drawn from the preforms.

(41) The optical fiber preform includes a core cane and one or more cladding layers that circumscribe the core cane. The preform is formed by processing a core-cladding assembly. Processing may include dehydrating, doping and/or sintering of the core-cladding assembly. The core-cladding assembly includes a core cane and a soot cladding monolith, where the soot cladding monolith is formed independent of the core cane and includes one or more porous cladding layers. The soot cladding monolith may include an internal cavity and the core-cladding assembly may be formed by inserting the core into the internal cavity. Sintering consolidates the porous cladding layers of the core-cladding assembly to achieve densified cladding layers that are fused to the core cane to form the optical fiber preform

(42) The optical fiber preforms are made from a cane-in-soot process that minimizes formation of defects. The optical fiber preforms are essentially defect free and can withstand reheating to fiber draw temperatures without cracking. In the cane-in-soot process, a core cane and soot cladding monolith are formed independent of each other. The core cane is a consolidated glass body. The soot cladding monolith is porous and includes an internal cavity. At least a portion of the core cane is inserted into the internal cavity to form a core-cladding assembly. The core-cladding assembly is an intermediate structure that is subsequently consolidated to form an optical fiber preform.

(43) Fabrication of the core cane may begin with formation of a core soot preform. FIG. 3 shows exemplary core soot preform 8 with handle 13. The core soot preform 8 may be formed using any suitable method, such as chemical vapor deposition (CVD) (e.g., outside vapor deposition (OVD), vapor axial deposition (VAD), modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD)) or any other suitable technique such as sol-gel processing or flame hydrolysis. The core soot preform 8 may be formed from pure silica or doped silica (for example, silica doped with a suitable dopant or dopants including, but not limited to, germania, boron, fluorine, aluminum, titanium, phosphorous, and/or chlorine). Doping may be used to control the refractive index of the core soot preform. The refractive index profile of the core cane may be a constant profile, a step index profile, or a monotonically varying profile (e.g. α-profile or super Gaussian profile). The core soot preform may be formed as a single layer or multilayer body, where the one or more layers may be doped, undoped and if doped, the one or more layers may include differences in the type, concentration, or distribution of dopants. The core soot preform 8 is a porous structure defining a plurality of interstices. The core soot preform 8 may include a passage extending the full length thereof from which a mandrel of the deposition apparatus has been removed. According to some embodiments, the core soot preform 8 has a density of no greater than about 1.0 g/cc, preferably of no more than about 0.7 g/cc, and more preferably of no more than about 0.6 g/cc

(44) The core soot preform 8 is consolidated to form a consolidated core glass preform and the consolidated core glass preform is drawn to form a core cane. The consolidation of the core soot preform 8 includes sintering and may include other process steps such as drying and/or doping. The consolidation of the core soot preform 8 may employ any suitable or desired processes or parameters consistent with the methods described herein. Suitable apparatus for consolidating the core soot preform 8 and drawing the consolidated core soot preform are known to those of skill in the art.

(45) The soot cladding monolith is formed independent of the core cane. By forming the soot cladding monolith independent of the core cane, the core cane is not exposed to water present as a reactant or byproduct in the deposition reaction or post-deposition processing of the soot cladding. Exclusion of water from the core cane leads to a reduction in hydroxyl content of the core portion of the fiber preform and in fibers drawn from the preform. As a result, attenuation at 1380 nm due to hydroxyl absorption is greatly reduced.

(46) The soot cladding monolith may be formed by depositing one or more layers of silica soot or doped silica soot on a substrate and removing the substrate to provide the soot cladding monolith. The substrate may be a bait rod. The cladding soot (or layers thereof) may be formed of pure silica or doped silica (for example, silica doped with a suitable dopant or dopants including, but not limited to, germania, boron, fluorine, aluminum, titanium, phosphorous, and/or chlorine). As indicated hereinabove, the cladding soot may include multiple layers that differ in doping and refractive index to provide fiber index profiles that include multiple cladding regions such as those shown in FIGS. 2A and 2B.

(47) In the following discussion, fabrication of a soot cladding monolith having three porous soot cladding layers is described. It is recognized, however, that the procedure outlined is generally applicable to a soot cladding monolith having any number of porous soot cladding layers.

(48) FIG. 4 illustrates deposition of a silica-based soot layer 112 on bait rod 120. The silica-based glass soot is formed by providing a vapor phase silica-based glass precursor material, such as SiCl.sub.4 or octamethylcyclotetrasiloxane (OMCTS), to a burner 122. The gas-fed burner 122 is supplied with fuel, such as H.sub.2, CH.sub.4, D.sub.2 (deuterium), CD.sub.4 or CO. Oxygen is also provided to burner 122 and the fuel and oxygen are combusted to create flame 126. In some embodiments, the vapor phase silica-based glass precursor material is SiCl.sub.4 and the gas-fed burner 122 is supplied with a non-hydrogenated fuel such as D.sub.2, CD.sub.4 or CO in order to limit the amount of residual OH in the deposited silica-based glass soot. The vapor phase silica-based glass precursor material may be delivered to the burner at a flow rate from about 4 L/min to about 10 L/min, while the fuel may be supplied to the burner at a flow rate from about 10 L/min to about 40 L/min.

(49) The vapor phase silica-based glass precursor material is reacted in the flame 126 to produce silica-based glass soot 128, which is deposited as soot layer 112 on bait rod 120 as the bait rod is rotated. The rotation rate may be from about 20 rpm to about 400 rpm, or preferably from 30 rpm to about 100 rpm. Soot layer 112 may have the same, higher, or lower refractive index than undoped silica. Higher or lower refractive indices may be achieved by supplying an updopant or downdopant precursor to burner 122. Soot layer 112 may constitute a single-layer soot cladding monolith or may constitute the innermost (smallest radius) layer of a multilayer soot cladding monolith. The flame 126 of the gas-fed burner 122 is traversed back and forth along the axial length of the bait rod 120 as indicated by arrow 124 as the bait rod is rotated thereby building up silica-based glass soot and forming soot layer 112 on the bait rod 120.

(50) FIG. 5 depicts deposition of soot layer 116 on soot layer 112. Soot layer 116 may be formed in a similar manner as soot layer 112. For example, a vapor phase silica-based glass precursor material, such as SiCl.sub.4 or OMCTS, may be supplied to the gas-fed burner 122 and reacted in the flame 126 to form silica-based glass soot which is deposited as soot layer 116 on soot layer 112 as the bait rod is rotated. Soot layer 116 may have the same, higher, or lower refractive index than soot layer 112. To achieve a preform that enables drawing of a fiber with the two-layer cladding region of FIG. 2A, for example, soot layer 112 may be undoped silica and soot layer 116 may be updoped silica. Soot layer 116 may constitute an outer layer of a two-layer soot cladding monolith or an intermediate layer of a soot cladding monolith having three or more layers.

(51) FIG. 6 depicts deposition of soot layer 114 on soot layer 116. Soot layer 114 may be formed in a similar manner as soot layer 112 or soot layer 116. For example, a vapor phase silica-based glass precursor material, such as SiCl.sub.4 or OMCTS, may be supplied to the gas-fed burner 122 and reacted in the flame 126 to form silica-based glass soot which is deposited as soot layer 114 on soot layer 116 as the bait rod is rotated. Soot layer 114 may have the same, higher, or lower refractive index than soot layer 116 or soot layer 112. To achieve a preform that enables drawing of a fiber with the three-layer cladding region of FIG. 2B, for example, soot layer 112 may be undoped silica, soot layer 116 may be downdoped silica, and soot layer 114 may be updoped silica. Soot layer 114 may constitute an outer layer of a three-layer soot cladding monolith or an intermediate layer of a soot cladding monolith having four or more layers. Additional layers of may be deposited similarly to obtain a soot cladding monolith having any desired number of layers.

(52) Process conditions used to form the different layers of a multilayer soot cladding monolith may be the same or different. Process variables include flame temperature, flow rates of precursors for silicon or dopants, traversal rate of the burner along the length of the bait substrate, and rotation rate of the bait substrate. Variations in process conditions can control the deposition rate of soot and density of soot in the as-deposited state. The flame temperature may be 1500° C. or higher. Higher flame temperatures promote higher as-deposited soot density. Conversely, lower flame temperatures lower as-deposited soot density.

(53) Soot density may also be influenced by the rate of traversal of the burner along the bait substrate. The traversal rate of the flame may be greater than 0.1 cm/sec, or greater than 0.25 cm/s or greater than 0.5 cm/s, or greater than 1 cm/s, or greater than 2 cm/s, or greater than 3 cm/s. Faster traversal rates may lead to less dense, more porous soot layers in the as-deposited state. Conversely, slower traversal rates may lead to denser, less porous soot layers in the as-deposited state. The density of the as-deposited soot layer may be less than 1.0 g/cm.sup.3, or less than 0.8 g/cm.sup.3, or less than 0.6 g/cm.sup.3, or less than 0.5 g/cm.sup.3. The density of the as-deposited soot layer may be greater than 1.0 g/cm.sup.3, or greater than 1.25 g/cm.sup.3, or greater than 1.5 g/cm.sup.3, or greater than 1.75 g/cm.sup.3 or greater than 2.0 g/cm.sup.3.

(54) The density of the as-deposited soot layer may also be influenced by the delivery rate of the soot precursor to the burner. The flow rate of silica-based soot precursors may be in the range from 0.1 L/min to 20 L/min. Lower flow rates promote the formation of soot layers having higher density in the as-deposited state. Conversely, higher flow rates promote the formation of soot layers having lower density in the as-deposited state.

(55) The density of the as-deposited soot layer may also be influenced by the rate of rotation of the bait rod during soot deposition. Decreasing the rate of rotation of the bait rod may assist in increasing the density of the as-deposited soot layer. Conversely, increasing the rate of rotation of the bait rod may assist in decreasing the density of the as-deposited soot layer.

(56) In one embodiment, the soot cladding monolith includes two soot layers, where the outer layer has a higher density than the inner layer. In another embodiment, the soot cladding monolith includes three soot layers, where an intermediate soot layer is positioned between an inner soot layer and outer soot layer and where the intermediate soot layer has a higher density than the inner soot layer. In still another embodiment, the soot cladding monolith includes four or more soot layers, where two or more intermediate soot layers are positioned between an inner soot layer and outer soot layer and where at least one of the intermediate soot layers has a higher density than the inner soot layer.

(57) Once the desired number and type of soot cladding layers has been deposited, the bait rod is removed to provide a soot cladding monolith. The space occupied by the bait rod forms an internal cavity of the soot cladding monolith.

(58) In the cane-in-soot process, the core cane is inserted in the internal cavity of the soot cladding monolith to form a core-cladding assembly.

(59) FIG. 7 shows placement of soot cladding monolith 110 in consolidation furnace 130. Soot cladding monolith 110 corresponds to the three-layer soot structure shown in FIG. 6 after removal of bait rod 120 and includes a series of concentric soot layers 112, 116, and 114 as described above. Soot cladding monolith 110 further includes internal cavity 118. In the embodiment of FIG. 7, internal cavity 118 extends through the entire length of soot cladding monolith 110. In other embodiments, internal cavity 118 extends only partially into soot cladding 110. Densified core cane 102 is inserted into internal cavity 118 to form a core-cladding assembly. The core-cladding assembly includes a gap between the outer surface of core cane 102 and the inner surface of soot cladding layer 112. The core-cladding assembly is processed in consolidation furnace 130.

(60) The core-cladding assembly is processed in accordance with the methods described herein to form an optical fiber preform. As noted hereinabove, the processing includes consolidation and effects a soot-to-glass transformation of the one or more porous soot cladding layers of the core-cladding assembly. Processing may include a pre-heat step, an intermediate heating step, and a sintering step, where one or more of the pre-heat step, intermediate heating step, and sintering step may include treatment of the soot cladding preform with a reducing agent. Processing of the soot cladding preform may also include a drying step and a doping step.

(61) In embodiments in which processing includes a drying step, the core-cladding assembly is treated with a drying agent. The drying step may be performed at a temperature of between about 800° C. and 1300° C., including in one or both of the pre-heating step and intermediate heating step described hereinabove. The drying agent penetrates the pores of the porous soot cladding layers of the core-cladding assembly and reacts with water or OH groups to remove water and OH groups from the porous soot cladding layers. The drying agent may also remove transition metals or other impurities that may be present in the porous soot cladding layers.

(62) Referring to FIG. 7, the drying agent may enter the gap between the outer surface of core cane 102 and the inner surface of soot cladding layer 112 as indicated by flow path 132. The drying agent may also enter pores within soot layers 112, 116, and 114, and may also surround or pass around the outer surface of soot cladding layer 114 as indicated by flow path 134. The flow rate of the drying agent through and/or around the core-cladding assembly may be in the range from about 1 L/min to about 40 L/min. The core-cladding assembly may be heated during dehydration. Heating may further the action of the drying agent and may promote removal of hydroxyl and water.

(63) The temperature of dehydration may be in the range from about 500° C. to about 1300° C. and the time of dehydration may be in the range from 30 min to 10 hours. The temperature of dehydration is preferably less than the temperature needed to initiate sintering of the soot cladding monolith component of the core-cladding assembly. Premature sintering of the soot cladding monolith closes pores and blocks access of the dehydration agent to the interior of the soot cladding monolith and thus inhibits removal of hydroxyl and water. The drying agent may be removed from the environment surrounding the soot cladding monolith and/or core-cladding assembly upon conclusion of dehydration.

(64) Suitable drying agents include chlorine-containing gases such as Cl.sub.2, SiCl.sub.4, GeCl.sub.4, SOCl.sub.2, and/or POCl.sub.3. The drying agent may optionally be diluted in an inert gas such as He, Ar, Ne and/or N.sub.2. In one embodiment, the dehydration gas comprises a mixture of 2% to 6% chlorine gas in helium gas. In some embodiments, the drying gas contains less than about 5 volume % chlorine, such as between about 0.01 and 3.0 volume % chlorine.

(65) In some embodiments, processing may include a doping step in which the core-cladding assembly is exposed to a dopant precursor. In one embodiment, the doping is performed during the pre-heat step of the soot-to-glass transformation process. In another embodiment, the doping is performed during the intermediate heating step. In still another embodiment, the doping is performed during the sintering step of the soot-to-glass transformation process. Doping preferably occurs while the cladding layers of the core-cladding assembly are sufficiently porous to permit diffusion or penetration of a dopant or doping precursor. In one embodiment, doping occurs after dehydration and before sintering.

(66) In one embodiment, doping is accomplished by supplying a doping precursor to a flow pathway between the outer surface of the core cane and inner layer of the soot cladding monolith in the core-cladding assembly. For example, a doping precursor may be supplied to flow pathway 132 shown in FIG. 7. In another embodiment, doping is accomplished by supplying a doping precursor to a flow pathway adjacent to the outer surface of the outer soot layer of the soot cladding monolith component of the core-cladding assembly. For example, a doping precursor may be supplied to flow pathway 134 shown in FIG. 7.

(67) When a doping precursor is supplied to porous soot layers of the soot cladding monolith component of the core-cladding assembly, it may enter the pores and deliver a dopant to the surface and/or throughout the interior of the soot layer. Doping may occur in multiple layers of the soot cladding monolith portion of the core-cladding assembly.

(68) Sintering of the core-cladding assembly may occur after dehydration. The drying agent and/or dopant precursor may be removed from the ambient of the core-cladding assembly before initiation of sintering.

(69) The sintering may consolidate the soot cladding monolith and fuse the soot cladding monolith with the core cane to form a consolidated fiber preform. During sintering, densification occurs as the pores of the soot cladding monolith collapse and the soot cladding monolith collapses and adheres to the core cane. The sintering temperature may be at least 1300° C., or at least 1350° C., or at least 1400° C., or at least 1450° C., or at least 1500° C. Higher sintering temperatures reduce the time of sintering.

(70) Sintering may be accomplished in a consolidation furnace by forming a hot zone of sufficient temperature to induce sintering within the consolidation furnace and exposing the core-cladding assembly to a thermal cycle in the furnace where the temperature may range from 1400° C. to about 1500° C. Sintering the core-cladding assembly produces an optical fiber preform. FIG. 8 shows optical fiber preform 100 formed by sintering the core-cladding assembly of FIG. 7. The optical fiber preform is a solid piece of glass with a central core with one or more concentric regions 104 and 106 surrounded by one or more concentric cladding regions 112, 114, and 116.

(71) An optical fiber can be drawn from the optical fiber preform. Suitable techniques and apparatus for drawing optical fiber from a consolidated glass preform are known to those of skill in the art. It will be appreciated that the consolidated glass core of the consolidated optical fiber preform will form the core (or a portion of the core) of the optical fiber and the consolidated cladding layers of the consolidated optical fiber preform will form the cladding portion of the optical fiber.

(72) Inspection of optical fiber preforms prepared by the cane-in-soot process reveals the presence of defects that may lead to failure of the preforms during draw. The defects include cracks, which can lead to fracturing of the optical fiber preform early in the process of reheating to the temperature needed to melt the preform for fiber draw. As noted hereinabove, fabrication of optical fiber preforms using the cane-in-soot process includes consolidation of a core-cladding assembly at high temperatures. The optical fiber preform formed in the consolidation process is cooled to room temperature and stored until the time of fiber draw.

(73) While not wishing to be bound by theory, it is believed that cooling of the optical fiber preform from the consolidation temperature to room temperature (or other lower temperature) leads to the formation of defects in the optical fiber preform. It is further believed that the defects originate from stresses that arise from differences in the coefficient of thermal expansion of the core cane and soot cladding monolith. As noted hereinabove, the core cane is typically formed from updoped silica, while the cladding is typically formed from undoped silica, downdoped silica, or a combination of undoped silica and downdoped silica. The compositional differences that exist between the materials used for the core cane and the soot cladding imply differences in thermal expansion coefficient that lead to differences in the rate or extent of contraction of the core and cladding regions of the optical fiber preform during the cooling process that follows consolidation. Differences in thermal contraction create stresses at the interface between the core cane and soot cladding monolith during cooling. It is believed that the stresses are sufficient to form defects, such as cracks or microcracks, in the optical fiber preform.

(74) Formation of cooling-induced defects is expected to be especially pronounced when a core cane having a high core-clad ratio is used to form the optical fiber preform in the cane-in-soot process. In a core cane with a high core-clad ratio, the radius of the updoped (high index) region is a significant fraction of the total radius of the core cane. The large radial extent of the updoped region positions the boundary of the updoped region in close proximity to a compositionally mismatched soot cladding assembly and increases the stresses arising from differential contraction of the core and cladding regions of the optical fiber preform during cool down. If the core-clad ratio is one, for example, the core is directly adjacent to the cladding and differences in thermal expansion coefficient at the interface between the core cane and cladding are most pronounced. When a core cane with a low core-clad ratio is used, in contrast, the updoped region is removed from the interface between the core cane and soot cladding monolith and is surrounded by an undoped material that more closely matches the thermal expansion coefficient of the cladding. As a result, thermal stresses arising from contraction are less pronounced and fewer defects form upon cooling.

(75) FIGS. 9(a)-(c) show an optical fiber preform formed from the cane-in-soot process using a core cane with flat end surfaces. FIG. 9(a) is a schematic showing optical fiber preform 140. Optical fiber preform 140 includes core cane 145 and consolidated cladding 150 supported by metal handle 155, which extends to terminal point 151. Handle 155 passes through entrance 153 to internal cavity 157 of consolidated cladding 150. Core cane 145 is situated in internal cavity 157 and has a generally cylindrical shape that includes flat end surface 147 that faces entrance 153 of internal cavity 157. Headspace 149 between end surface 147 and terminal point 151 of handle 155 is occupied by air and/or gases present in the processing environment. Junction 159 constitutes a three-way interface between consolidated cladding 150, core cane 145, and the gases present in headspace 149.

(76) FIGS. 9(b) and 9(c) show images of an optical fiber preform consistent with the schematic shown in FIG. 9(a). The preform was prepared by the cane-in-soot process. The image shown in FIG. 9(b) is an image of the preform at elevated temperature (1050° C.) after consolidation and before cooling. The image shown in FIG. 9(b) indicates that no appreciable concentration of defects was present in the preform before cooling. The image shown in FIG. 9(c) is an image of the preform after cooling to room temperature. The image shown in FIG. 9(c) shows the presence of defects (e.g. check defects or “checks”) in the cooled preform. When optical fiber preforms having check defects are reheated to the draw temperature for drawing fibers, the optical fiber preform may fracture due to shearing, crack propagation or other forces arising from the defects. Fracture leads to failure of the optical fiber preform and necessitates termination of the drawing process.

(77) The origin of the defects that develop during cooling of cane-in-soot preforms is believed to be due to a mismatch in the coefficient of thermal expansion of the core and cladding regions of the preform. As noted hereinabove, the core is typically an updoped silica material and the cladding is typically undoped silica or a combination of undoped silica and downdoped silica. Ge is a common updopant for the core region and the coefficient of thermal expansion of Ge-doped SiO.sub.2 is known to be significantly higher than the coefficient of thermal expansion of undoped silica or downdoped silica. FIG. 10 shows the average coefficient of thermal expansion α.sub.100-500 over the temperature interval from 100° C. to 500° C. as a function of relative refractive index Δ % for Ge-doped silica, undoped silica, and F-doped silica. The relative refractive index Δ % is a measure of refractive index relative to undoped silica and is proportional to doping concentration. Positive values of Δ % signify updoped forms of silica, negative values of Δ % signify downdoped forms of silica, and Δ %=0 corresponds to undoped silica. For purposes of FIG. 10, Ge is included as an updopant and F is included as a down dopant. FIG. 10 shows that the coefficient of thermal expansion of Ge-doped silica increases with increasing Ge doping concentration and is higher than the coefficient of thermal expansion of both undoped silica and F-doped silica.

(78) For Ge-doped silica, the coefficient of thermal expansion (CTE) (in units of 1/° C.) and Δ % are related to GeO.sub.2 concentration [GeO.sub.2] (in units of weight percent) through
CTE=(5.05+0.42075[GeO.sub.2])×10.sup.−7  Eq. 2
and
Δ%=0.055[GeO.sub.2]  Eq. 3

(79) FIG. 11 shows the radial dependence of the coefficient of thermal expansion at room temperature calculated from Eq. 2 of an optical fiber preform having a Ge-doped silica core and an undoped silica cladding. The Ge-doped silica core has a radius of 5 mm and a core-clad ratio of 0.9. The Ge dopant distribution conformed to a step index-profile having a peak dopant concentration of 6.5 wt % at the centerline position of the core (r=0) and remaining substantially flat out to r=4.5 mm. The cladding was undoped silica with an inner radius of 5 mm and an outer radius of 65 mm. The cladding was in direct contact with the core. FIG. 11 indicates that the coefficient of thermal expansion is significantly higher in the core region than in the cladding region of the preform. The peak coefficient of thermal expansion in the core region is 8.42×10.sup.−7/° C., while the coefficient of thermal expansion of undoped silica is 5.05×10.sup.−7/° C. Although the concentration of Ge decreases with radius in the core region, the coefficient of thermal expansion in the outer radial portions of the core remains appreciably higher than the coefficient of thermal expansion of undoped silica.

(80) FIG. 12 shows the calculated radial tensile stress distribution in end portion 210 of a consolidated preform made by the cane-in-soot process after cooling to room temperature. End portion 210 is the portion in the vicinity of upper boundary 215 of the core cane. End portion 210 includes core cane 220 and cladding 240. Core cane 220 is cylindrical in shape and has a diameter 230 of 10 mm. Core cane 220 is made from silica doped with GeO.sub.2 and has a core-clad ratio of 0.9. The doping profile of GeO.sub.2 is a step index with a peak concentration of 6.5 wt % at the centerline position of the core (r=0) and remaining substantially flat out to r=4.5 mm. The coefficient of thermal expansion of the core cane of GeO.sub.2-doped silica increases with increasing doping concentration. The peak thermal expansion coefficient of core cane 220 is 7.97×10.sup.−7/° C. at the centerline (r=0) position and remaining flat out to r=4.5 mm before decreased to 5.05×10.sup.−7/° C. at r=4.5 mm. Cladding 240 is made from undoped silica and had an inner radius of 5 mm and an outer radius of 65 mm. Only the portion of cladding 240 proximate to core cane 220 is depicted in FIG. 12. Cladding 240 has an annular shape and surrounds core cane 220. Cladding 240 has a thermal expansion coefficient of 5.05×10.sup.−7/° C. End portion 210 also includes unoccupied space 250 that permits expansion of core cane 220 in the axial direction during formation of the preform.

(81) The radial tensile stress that develops during cooling of the preform during consolidation is depicted in FIG. 12. Dark regions are regions of low radial tensile stress and light regions are regions of high radial tensile stress. Regions of high radial tensile stress are present in the central doped portion of core cane 220 and in encircled corner regions 225 and 235. The undoped outer radial portion of core cane 220 exhibits low radial tensile stresses. Corner regions 225 and 235 are at the junction of the interface of core cane 220, cladding 240, and overhead gas in unoccupied space 250. The overhead gas may be air, a process gas, an inert gas, or other gas. The high radial tensile stresses present at corner regions 225 and 235 are believed to lead to defects in the preform that cause fracture upon reheating to draw temperature.

(82) FIG. 13 shows the calculated radial tensile stress at the corner regions of the preform as a function of corner radius for the preform shown in FIG. 12. Corner radius is a measure of the shape of the surface of end portion 210 in the transition region from core cane 220 to cladding 240. Large corner radius corresponds to a smooth, gradual transition across the interface from core cane 220 to cladding 240, while a small corner radius signifies an abrupt, more discontinuous transition across the interface from core cane 220 to cladding 240. FIG. 13 shows that radial tensile stress decreases with increasing corner radius. Under cooling rates from the consolidation temperature that are typically used in the manufacture of the preform, the corner radius of the preform is below 0.1 mm and often below 0.01 mm. The radial tensile stress at the corner region is accordingly high and likely sufficient in magnitude to induce the formation of cracks or other defects in the preform. FIG. 14 shows an image of a Ge-doped silica core cane having a corner region (encircled region) with defects.

(83) The present disclosure provides a strategy for minimizing radial tensile stresses in the corner region(s) of consolidated preforms made in the cane-in-soot process. The strategy includes placement of a cap at one or more corner regions, where the capping material has a coefficient of thermal expansion less than the coefficient of thermal expansion of the core cane. In one embodiment, the coefficient of thermal expansion of the core cane is greater than the coefficient of the cladding, while the coefficient of thermal expansion of the cap is less than the coefficient of thermal expansion of the core cane and greater than the coefficient of thermal expansion of the cladding. In another embodiment, the coefficient of thermal expansion of the core cane is greater than the coefficient of the cladding, while the coefficient of thermal expansion of the cap is equal to the coefficient of thermal expansion of the cladding. In still another embodiment, the coefficient of thermal expansion of the core cane is less than the coefficient of the cladding, while the coefficient of thermal expansion of the cap is greater than the coefficient of thermal expansion of the core cane and less than the coefficient of thermal expansion of the cladding. In another embodiment, the coefficient of thermal expansion of the core cane is less than the coefficient of the cladding, while the coefficient of thermal expansion of the cap is equal to the coefficient of thermal expansion of the cladding.

(84) The cap(s) may be integrated with the core cane before or after insertion of the core cane into the internal cavity of the soot cladding monolith to form the core-cladding assembly. The integrated cap(s) may be in direct contact with the core cane, rigidly joined to the core cane, or in indirect contact with the core cane. In one embodiment, the cap is formed on the end of the core cane during production of the core cane and the capped core cane is inserted into the internal cavity of the soot cladding monolith in the cane-in-soot process. The cap may be formed on the core cane by direct deposition onto the end surface(s) of the core cane or by fusing an independently formed cap on the core cane before integration of the core cane with the soot cladding monolith to form the core-cladding assembly. The core cane may be capped on two or more ends. In one embodiment, the core cane has a cylindrical shape and is capped on one or both end surfaces. When two or more caps are integrated with the core cane, they may have the same or different composition and/or the same or different size, shape and/or coefficient of thermal expansion.

(85) In another embodiment, an uncapped core cane is inserted into the internal cavity of the soot cladding monolith and one or more caps are also placed in the internal cavity. The one or more caps may be placed directly on one or more end surfaces of the core cane. Alternatively, an intervening material may be placed between the end surface of the core cane and the cap to hold the cap or to facilitate integration of the cap with the core cane during consolidation of the core-cladding assembly in the process of forming the preform. The intervening material may be a flux material or an adhesive material and may have a composition that is the same as or different from the composition of either or both of the core cane and capping material. The intervening material may be consolidated or unconsolidated. In one embodiment, the intervening material is unconsolidated SiO.sub.2 soot powder. In another embodiment, the intervening material is an SiO.sub.2 soot pellet. Fusion and integration of the cap with the core cane occurs during the heating processes that occur during formation of the preform from the core-cladding assembly in the cane-in-soot process. In one embodiment, fusion and integration of the cap with the core cane occurs during the sintering and/or consolidation processes of the cane-in-soot processes.

(86) FIG. 15 shows capped core cane 300 in cross section. Capped core cane 300 includes core cane 310 and caps 305 and 315. Core cane 310 has a cylindrical shape and caps 305 and 315 are positioned at end surfaces of core cane 310. In the embodiment of FIG. 15, caps 305 and 315 are formed directly on the end surfaces. Alternatively, an intervening material (not shown) may be positioned between cap 305 and core cane 310 and/or between cap 315 and core cane 310. Caps 305 and 315 are in direct contact with core cane 310. Caps 305 and 315 may be rigidly joined to core cane 310.

(87) FIG. 16 shows assembly of capped core cane 300 from core cane 310, cap 305, and cap 315. In this embodiment, core cane 310, cap 305, and cap 315 are formed independently and combined to form capped core cane 300. An intervening material (not shown) may be positioned between cap 305 and core cane 310 and/or between cap 315 and core cane 310.

(88) FIG. 17 shows formation of a core-cladding assembly. Capped core cane 300 is inserted into internal cavity 325 of soot cladding monolith 330 to form core-cladding assembly 335. Soot cladding monolith 330 and core-cladding assembly 335 are shown in cross-section. Soot cladding monolith 330 includes internal cavity 325 within annular porous soot region 320. Capped core cane 300 can be formed, for example, by the methods described hereinabove in connection with FIGS. 15 and 16. As noted hereinabove, an intervening material may be included between an end surface of core cane 310 and cap 305 and/or between an end surface of core cane 310 and cap 315.

(89) FIG. 18 shows an alternative method for forming core-cladding assembly 335. Core cane 310 is inserted into internal cavity 325 of soot cladding monolith 330. Caps 305 and 315 are next inserted into internal cavity 325 adjacent to the end surfaces of core cane 310 to form core-cladding assembly 335. As noted hereinabove, an intervening material may be included between an end surface of core cane 310 and cap 305 and/or between an end surface of core cane 310 and cap 315.

(90) FIG. 19 shows consolidation of core-cladding assembly 335 to form preform 345. Preform 345 includes core cane 310 with caps 305 and 315 within internal cavity 325. Internal cavity 325 is circumscribed by annular consolidated soot region 340. Consolidated soot region 340 is a fully densified glass. As noted hereinabove, an intervening material may be included between an end surface of core cane 310 and cap 305 and/or between an end surface of core cane 310 and cap 315.

(91) The cap(s) may have a cross-sectional dimension that closely corresponds to the cross-sectional dimension of the core cane. If, for example, the core cane has a cylindrical shape, the cross-sectional dimension of the core cane is a diameter and the cap(s) may have a cylindrical shape with a diameter that matches or closely matches the diameter of the core cane. The cross-sectional dimension of the core cane and/or cap(s) may also closely correspond to the cross-sectional dimension of the internal cavity of the soot cladding monolith.

(92) The thickness of the cap is the direction normal to the cross-sectional dimension of the cap. The present inventors have discovered, unexpectedly, that suppression of defect formation in the preform by the cap(s) can be achieved even with relatively thin cap(s). FIG. 20 shows the calculated variation in radial tensile stress at a corner region (such as corner regions 225 and 235 shown in FIG. 12) of a preform made by the cane-in-soot process as a function of cap height (thickness). The basis of the calculation shown in FIG. 20 is a preform formed by consolidating a core-cladding assembly having a GeO.sub.2-doped silica core cane with SiO.sub.2 caps and a silica soot cladding monolith. The GeO.sub.2-doped silica core cane had a diameter of 10 mm and a core-clad ratio of 0.9. The SiO.sub.2 caps were positioned directly on the end surfaces of the GeO.sub.2-doped silica core cane. The SiO.sub.2 caps had a diameter of 10 mm and variable height (thickness) as indicated in FIG. 20. The corner radius of the preform was 0.2 mm.

(93) The results shown in FIG. 20 indicate that radial corner tensile stress decreases significantly with increasing height (thickness) of the SiO.sub.2 cap. The data point at zero height (thickness) corresponds to a preform with an uncapped core cane. The radial corner tensile stress of the preform without an SiO.sub.2 cap was 102 MPa. The radial corner tensile stress decreased to 60 MPa, 51 MPa, and 17 MPa by including a SiO.sub.2 caps having heights (thicknesses) of 1 mm, 2 mm and 5 mm, respectively. Reduction of the radial corner tensile stress reduces the likelihood of defect formation in the preform upon cool down during the consolidation process.

(94) The radial tensile stress at the corner region of the preform may be less than 100 MPa, or less than 80 MPa, or less than 60 MPa, or less than 30 MPa, or less than 20 MPa, or less than 10 MPa, or between 5 MPa and 100 MPa, or between 5 MPa and 80 MPa, or between 5 MPa and 60 MPa, or between 5 MPa and 30 MPa, or between 5 MPa and 20 MPa, or between 10 MPa and 60 MPa, or between 10 MPa and 50 MPa, or between 10 MPa and 40 MPa, or between 10 MPa and 30 MPa, or between 10 MPa and 20 MPa.

(95) The height (thickness) of a cap integrated with a core cane may be less than 100 mm, or less than 50 mm, or less than 25 mm, or less than 10 mm, or less than 5 mm, or less than 3 mm, or less than 1 mm, or in the range from 0.5 mm to 100 mm, or in the range from 1.0 mm to 75 mm, or in the range from 2.0 mm to 60 mm, or in the range from 5.0 mm to 50 mm, or in the range from 8.0 mm to 40 mm, or in the range from 10 mm to 35 mm.

(96) FIG. 21 is an image of a preform made by consolidating a core-cladding assembly that included a capped core cane. The locations of the core cane and cap at the upper end of the preform are noted. The core cane was GeO.sub.2-doped silica having a diameter of 10 mm and a core-clad ratio of 0.9. The cap was undoped silica having a diameter of 10 mm and a height (thickness) of 2 mm. The upper surface of the cap is marked with a dotted line and the corner regions are encircled. The image indicates that the corner regions are essentially free of defects.

(97) FIG. 22 shows the calculated radial tensile stress of a preform made by consolidating a core-cladding assembly having a GeO.sub.2-doped silica core cane capped with a 5 mm thick SiO.sub.2 end cap. Preform 360 includes GeO.sub.2-doped silica core cane 365 having diameter 362 of 10 mm, silica cladding 370, and silica end cap 375 having a height (thickness) of 5 mm. Encircled corner regions 380 and 385 are shown as is unoccupied space 390. The magnitude of radial tensile stress is indicated by coloring where dark coloring signifies low radial tensile stress and light coloring signifies high radial tensile stress. The radial tensile stress profile shown in FIG. 20 indicates that inclusion of end cap 375 significantly reduces radial tensile stress at corner regions 380 and 385 relative to the uncapped analog shown in FIG. 12. High radial tensile stresses are limited to the internal doped region of core cane 365 and the interface between the end surface of core cane 365 and end cap 375.

(98) Inclusion of one or more caps on the core cane alleviates the development of radial tensile stresses at the core-cladding-gas junctions associated with corner regions 380 and 385 and accordingly reduces the likelihood of defect formation in the preform (as shown, for example, in FIG. 21). A theoretical model for a preform consolidated from a core-cladding assembly having a GeO.sub.2-doped silica core with diameter 10 mm and core-clad ratio 0.9 and a silica cladding indicates that the radial corner tensile stress of the preform is ˜80 MPa and the failure probability of the preform during draw (probability of shearing of the preform during draw due to defects in the preform caused by the radial corner tensile stress) is ˜90%. The model predicts that the radial corner tensile stress can be reduced to ˜45 MPa and that the failure probability is reduced to ˜3% by including a 1 mm thick SiO.sub.2 cap. With a 2 mm thick SiO.sub.2 cap, the radial corner tensile stress is reduced to ˜32 MPa and the failure probability is reduced to less than 1%.

(99) The reduction of radial corner tensile stresses available from the present disclosure enables production of preforms from core canes having a large core-clad ratio without significant development of defects during cooling of the preform in the consolidation process. The core-clad ratio of the core cane may be greater than 0.30, or greater than 0.40, or greater than 0.50, or greater than 0.60, or greater than 0.70, or greater than 0.80, or greater than 0.85, or greater than 0.90, or greater than 0.95, or in the range from 0.30 to 1.0, or in the range from 0.60 to 0.95, or in the range from 0.65 to 0.95, or in the range from 0.70 to 0.95, or in the range from 0.75 to 0.95, or in the range from 0.80 to 0.95, or in the range from 0.60 to 0.90, or in the range from 0.65 to 0.90, or in the range from 0.70 to 0.90, or in the range from 0.75 to 0.90, or in the range from 0.80 to 0.90.

(100) Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

(101) It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the illustrated embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments that incorporate the spirit and substance of the illustrated embodiments may occur to persons skilled in the art, the description should be construed to include everything within the scope of the appended claims and their equivalents.