WO2006083354A2

WO2006083354A2 - Low-water optical fiber preform and process for making it

Info

Publication number: WO2006083354A2
Application number: PCT/US2005/042079
Authority: WO
Inventors: Arnab Sarkar; Bedros Orchanian; Kim Nhu Vo
Original assignee: Nextrom Holding, S.A.
Priority date: 2004-11-18
Filing date: 2005-11-17
Publication date: 2006-08-10
Also published as: WO2006083354A3

Abstract

An improved flame-hydrolysis process is disclosed, for manufacturing large optical fiber core preforms, useful in the production of low water, single-mode optical fibers. In one feature of the invention, the preform is manufactured using a vapor axial deposition (VAD) apparatus to form an inner core and an outside vapor deposition (OVD) apparatus to form an outer core, wherein the VAD and OVD apparatus both are located within the same housing. In another feature of the invention, the concentration of a germanium dopant in the inner and outer cores is substantially uniform, but increases immediately adjacent the outer surface of the outer core. Further, the concentration of a fluorine dopant in a surrounding cladding layer decreases from a maximum value immediately adjacent to the outer core. These variations in dopant concentrations are selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the preform is subsequently being processes.

Description

Low- Water Optical Fiber Preform and Process for Making It

Background of the Invention

1. Field of the Invention

This invention relates to the manufacture of optical fiber preforms, by a chemical vapor deposition process called flame hydrolysis, in which vapors of glass-forming materials are fed through a water-generating flame, which reacts with the vapors to form small particles of glass, called soot, a portion of which is then collected as a porous body, dehydrated, and sintered to form a dense glass preform from which an optical fiber can be drawn.

2. Description of the Prior Art

The basic flame-hydrolysis process is described in U.S. Patent No. 2,272,342, which issued in 1942. Two distinct flame-hydrolysis processes for producing porous glass preforms are described in detail in chapters 2 and 3 of a book entitled "Optical Communications, Volume 1, Fiber Fabrication," edited by Tingye Li (1985). The two processes are commonly described as outside vapor deposition (OVD) and vapor axial deposition (VAD). In both of these processes, the glass preform is produced in two steps, with the first step producing a core preform, which includes a core and a part of the cladding glass of the resultant fiber.

The OVD process was described first in U.S. Patent No. 3,737,292 and more recently in U.S. Patent Nos. 4,413,882; 4,486,212; 4,453,961 ; and 6,477,305. In this process, the core and part of the cladding material are first deposited onto a removable, tapered ceramic mandrel, after which the mandrel is removed and the deposited porous soot body is dehydrated and sintered into a tubular core preform. The two ends of the tubular preform then are sealed under vacuum, and the preform then is elongated to form core rods for further processing into optical fibers.

The VAD process was first described in U.S. Patent No. 3,966,446 and more recently in U.S. Patent Nos. 4,367,085, 4,419,116, and 6,131,415. In this process, the core is grown axially on the tip of a glass handle, while simultaneously additional burners deposit the cladding glass on the core preform. After the porous soot body has grown to a desired length, it is removed from the deposition machine, dehydrated, and sintered into dense glass core preform, which is then elongated and processed into an optical fiber.

the past to combine the advantageous features of the OVD and VAD processes. For example, U.S. Patent Nos. 4,599,098, 5,028,246, and 5,558,693 disclose processes in which the core is made by the VAD process and the cladding is made by the OVD process. However, for various reasons, the processes taught in these patents have not been used commercially for in mass production of single-mode fibers.

Both VAD and OVD processes are commercial processes, but suffer from limitations that prevent manufacturing core preforms of long length and large diameter.

In the OVD process, a slight friction is present between the mandrel and the deposited glass, even though an amorphous carbon material is initially deposited onto the mandrel as a release agent, and even though the ceramic mandrel has a higher expansion coefficient than does the glass deposited onto it. This friction limits the length of the core preform that can be made using the OVD process. A second limitation of the OVD process relates to an inability in practice to prevent recontamination of the inner surface of the tubular core preform, making it difficult to consistently produce low- water single mode optical fibers.

One limitation of the VAD process relates to difficulties in producing large diameter core preforms, because of the requirement of density control of the porous preform.

A further limitation of both the OVD process and the VAD process is that during the dehydration and sintering steps of the process, some of the dopant deposited in the core to raise its refractive index (most commonly germanium dioxide) diffuses into the clad glass. This creates a diffused refractive index interface between the preform' s core and clad glass, which in turn adversely affects the dispersion properties of the resultant optical fiber. This problem is more acute in the VAD process, because of the limitations of tailoring the density profile in the core/cladding interface.

It should, therefore, be appreciated that there remains a need for an improved core preform, and a process for making it, which has a long length and a large diameter, with a sharp refractive index profile at its core/cladding interface and in a form that is conveniently dehydrated and sintered and subsequently processed into low water-peak single-mode optical fibers. The present invention fulfills this need. The invention resides in a cylindrical optical fiber preform, and a process for making it, wherein the preform exhibits a sharp refractive index profile at its core/cladding interface. More particularly, in one aspect of the invention, the process includes steps of (1) forming an inner core using a vapor axial deposition apparatus located within a housing; (2) forming an outer core over the inner core using an outside vapor deposition apparatus located within the same housing; and (3) forming a cladding layer over the outer core using the outside vapor deposition apparatus. These three forming steps preferably all are performed while the optical fiber preform being made remains within the housing.

In a separate and independent aspect of the invention, the inner core and the outer core both are doped with germanium, and the concentration of the germanium dopant is substantially uniform in the inner core and in a substantial portion of the outer core, but such concentration has an increasing gradient adjacent to the outer surface of the outer core. Further, the cladding layer is an interface cladding layer doped with fluorine, at a concentration having a decreasing gradient from a maximum value adjacent to the outer core. The increasing gradient in the concentration of the germanium dopant in the outer core,^' and the decreasing gradient in the concentration of the fluorine dopant in the interface cladding layer, can be selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the optical fiber preform is subsequently being processed.

In another, more detailed feature of the invention, the process can further include a step of forming an outer cladding layer (e.g., of substantially pure silica glass) over the interface cladding layer. This outer cladding layer can be formed using the outside vapor deposition apparatus.

The invention also is embodied in a cylindrical optical fiber preform that is made using the process described briefly above.

Other features and advantages of the invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. Figure 1 is a cross-sectional view of an optical fiber preform embodying the present invention.

Figure 2a is a schematic drawing depicting the concentration profile of germanium dioxide of the optical fiber preform of Figure 1 , after deposition but before sintering.

Figure 2b is a schematic drawing depicting the concentration profile of germanium dioxide of the optical fiber preform of Figure 1, after sintering.

Figure 3 is a schematic drawing depicting the refractive index profile of the optical fiber preform of Figure 1.

Figure 4a is a cross-sectional view a combination VAD/OVD burner traverse apparatus suitable for use in producing the optical fiber preform of Figure 1, showing the apparatus configured during VAD deposition of the preform' s inner core region 11.

Figure 4b is a cross-sectional view of the same combination VAD/OVD burner traverse apparatus as depicted in Figure 4a, but showing the apparatus configured during sequential OVD deposition of the preform' s outer core region, interface clad region, and outer clad region.

Description of the Preferred Embodiments and Processes

Figure 1 is a cross-sectional view of a glass preform 10 constructed using the process of the invention. The preform includes a two-part core and a two-part cladding. The two-part core includes an inner core region 11, called an integral bait, which is produced using a vapor axial deposition (VAD) process, and it further includes an outer core region 12, which is produced layer-by-layer using an outside vapor deposition (OVD) process. The two-part cladding includes an interface cladding region 13, which is produced using the OVD process, where the concentration of germanium dioxide is tailored to minimize its diffusion into the core, and it further includes an outer cladding region 14, which contains the bulk of the cladding glass. The outer cladding region has a thickness, t, that is at least four times thicker than the core radius. The relatively large size of the cladding ensures that any water adsorbed onto the outside surface of the preform does not substantially affect the water content of the resultant fiber. ipi Ii

¹¹ '

the composition profile of the preform 10 after deposition. The solid line 15 indicates the concentration of germanium dioxide in the inner core region 11 and the outer core region 12. No germanium dioxide is deposited in the interface cladding region 13 or in the outer cladding region 14. Particularly significant is the slight increasing gradient of germanium dioxide content in the outer core region 12, immediately adjacent to the interface cladding region 13. The broken line 16 shows the concentration of fluorine in the interface cladding region 13 after deposition, and in particular a decreasing gradient immediately adjacent to the outer core region 12.

Sintering, which occurs at elevated temperatures, can cause some of the germanium dioxide to diffuse, or migrate, within the core regions 11 and 12 and also to diffuse out of the outer core region 12 into the interface cladding region 13. Figure 2b depicts the composition profile of germanium dioxide in the preform 10, with the solid line 15' showing the change in the germanium dioxide composition profile after sintering. The broken line 16' shows that the fluorine composition profile after sintering remains substantially unchanged.

Figure 3 shows the refractive index profile of the inner core region 11, the outer core region, 12, and the interface cladding region 13, both with and without fluorine doping of the interface cladding region. Line 17 is a typical refractive index profile without fluorine doping, and line 18 is the refractive index profile with fluorine doping, in accordance with the present invention.

Those skilled in the art will appreciate that the composition profiles depicted in

Figures 2a and 2b, and the refractive index profiles depicted in Figure 3, are not precise or unique. Depending on the temperature gradient of the VAD core deposition surface, the germanium dioxide concentration within the core can vary from the depicted values. Further, during layer-by-layer OVD deposition, the germanium dioxide concentration within layers can vary and cause striations in refractive index visible even to the naked eye. However, showing these well-known artifacts in the figures is not central to the description of the invention. Similarly, after diffusion, the germanium dioxide concentration within the inner core region 11 varies depending on the variation of the density of the core preform. These variations also are well known and are not central to the description of the invention.

Figures 4a and 4b are two views of a combination VAD/OVD burner traverse apparatus suitable for use in producing the single-mode optical fiber preform 10 in accordance

sy'ecSc'aliy, Figure 4a shows the apparatus as configured during VAD deposition of germanium dioxide-doped silica to form the inner core region 11. Figure 4b shows the apparatus as configured during sequential OVD deposition of (1) the germanium dioxide-doped silica outer core region 12, (2) the fluorine-doped silica interface cladding region 13, and (3) the pure silica outer cladding region 14.

As shown in Figures 4a and 4b, the combination VAD/OVD burner traverse apparatus includes a lower deposition chamber 19 that houses a VAD core burner 25. A rotating and traversing preform chuck 23 holds a VAD handle 24. As the germanium-doped silica particles are deposited onto the handle, a laser position control system (not shown in the figures) moves the preform chuck 23 upwards, such that the burner deposits a porous soot cylinder. This forms a soot mandrel that constitutes the inner core region 11, as shown in Figure 1. One suitable laser position control system is disclosed in U.S. Patent No. 4,062,665.

¹ The apparatus shown in Figures 4a and 4b further includes an upper deposition chamber 20, which constitutes a burner-traverse OVD deposition chamber, with multiple OVD burners 26 capable of traversing the length of the chamber. A linear vertical slot in the deposition chamber exhausts uncollected reaction products into an exhaust duct 21. A laminar forced airflow system 22, located behind the deposition burners, creates airflow across the deposition chamber into the exhaust duct 21.

Those skilled in the art will appreciate that the preform, alternatively, could be produced sequentially using separate conventional VAD and OVD machines, hi such case, care must taken to ensure that transferring the inner core region 11 from the VAD machine to the OVD machine does not damage the inner core region's outside surface. Further, the OVD machine could be a preform-traverse machine, in which the inner core region 11 produced by VAD machine could traverse across stationary OVD burners, to deposit the OVD layers. However, in such a case, the machine height would need to be much higher. The OVD layers also could be deposited using an oscillating burner OVD machine, as taught in U.S. Patent No. 5,211,732. Thus, it will be appreciated that the present invention contemplates these and other modifications and variations of the apparatus.

The process for making the preform 10 is described as follows. The inner core region 11 is deposited using a conventional VAD process. During its deposition, oxy-hydrogen or oxy-natural gas flame gases, along with silicon tetrachloride and germanium tetrachloride (or simi&Wlaώe^lask-fόWiitfδϊϊώtlof the cations) are fed through different streams of a specially designed flame-hydrolysis burner 25. The burner can have a concentric tube design made of a suitable metal or quartz. Alternatively, the gases can be fed through concentric rings of apertures of suitable dimensions. Those skilled in the art know that these burners have evolved from the 1940s, when the flame-hydrolysis process was first invented, and that the burners can be of many different variations and yet be effective in the deposition process.

During the subsequent OVD deposition process, the outer core region 12 is formed by feeding germanium tetrachloride and silicon tetrachloride (or similar suitable glass- forming reactants of the cations) into one or more OVD flame-hydrolysis burners. These burners traverse on one or more burner carriages, at a suitable traverse speed. As the final layers of the outer core region 12 are being deposited, the relative concentration of germanium tetrachloride is progressively increased, so as to provide the desired increase in the deposited germanium dopant, as indicated in Figure 2a.

After the outer core region 12 has been deposited, the germanium tetrachloride flow is terminated and fluorine doping is performed, instead, to produce the interface cladding region 13. Specifically, a fluorine-containing vapor or gas, from a family of materials like SiF₄, Si₂F₆, CF₄, and SF₆, is fed into the flame-hydrolysis burner, as disclosed in U.S. Patent No. 4,161,505. During this deposition process, the quantity of fluorinating gas is progressively reduced, layer by layer, to provide the desired fluorine concentration indicated in Figure 2a. The depression in index of refraction caused by fluorine doping will offset the rise in refractive index in this layer caused by diffusion into it of germanium dioxide from the outer core region 12 during the subsequent dehydration and sintering process steps.

Following the deposit of the interface cladding region 13, the deposited porous soot preform is dehydrated in a controlled atmosphere of chlorine and helium, at a temperature of about 1200° C, and it is then zone-sintered at about 1500° C, in a helium- and fluorine-containing gas environment, to sinter the preform into a dense glass body. This causes the entire two-part core to be slightly fluorine doped during sintering, but this also minimizes the possibility of re- incorporating moisture from the helium gas during the sintering step. The fluorine-doped interface cladding region 13 sinters at a lower temperature, and thus sooner, than do the germanium-doped inner core region 11 and outer core region 12. This has the beneficial result of inhibiting excessive migration of the germanium dopant outwardly into the interface cladding region 13. The resulting glass preform is elongated and additional cladding glass is deposited and limiαlfaneόusTy ^ in a chlorine-containing helium atmosphere, to form the glass preform.

Those skilled in the art will appreciate that numerous variations of the process steps described above can be used to produce an optical fiber preform in accordance with the invention. For example, the two-part core preform can be inserted into a suitable tube and the tube collapsed onto the core preform under vacuum and high temperature, to form the final preform. Additional cladding glass can be added by a sequence of elongation, deposition, and sintering, to form the final preform. Also the core preform can be directly drawn into a fiber in a draw operation, where the preform is inserted into a silica glass cylinder. Thus, it will be appreciated that the present invention contemplates these and other modifications and variations. Accordingly, the invention is defined only by the following claims.

Claims

1. A process for making a cylindrical optical fiber preform, comprising:

forming an inner core using a vapor axial deposition apparatus located within a housing; forming an outer core over the inner core using an outside vapor deposition apparatus located within the same housing; and forming a cladding layer over the outer core using the outside vapor deposition apparatus.

2. A process as defined in claim 1 , wherein the steps of forming an inner core, forming an outer core, and forming a cladding layer are performed while the optical fiber preform being made remains within the housing.

3. A process as defined in claim 1, wherein:

the inner core and the outer core, formed in the first two steps of forming, both are doped with germanium; and the concentration of the germanium dopant is substantially uniform in the inner core and in a substantial portion of the outer core, but such concentration has an increasing gradient adjacent to the outer surface of the outer core.

4. A process as defined in claim 3, wherein:

the cladding layer formed in the step of forming a cladding layer is an interface cladding layer doped with fluorine; and the concentration of the fluorine dopant in the interface cladding layer has a decreasing gradient from a maximum value adjacent to the outer core.

5. A process as defined in claim 4, wherein:

the process further comprises forming an outer cladding layer over the interface cladding layer; and the outer cladding layer comprises substantially pure silica glass.

6. A process as defined in claim 5, wherein forming an outer cladding layer is performed using the outside vapor deposition apparatus.

7. A process as defined in claim 4, wherein the increasing gradient m the concentration of the germanium dopant in the outer core, and the decreasing gradient in the concentration of the fluorine dopant in the interface cladding layer, are selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the optical fiber preform is subsequently being processed.

8. A process for making a cylindrical optical fiber preform, comprising:

forming a core that is doped with germanium, wherein the concentration of the germanium dopant is substantially uniform throughout the core, but has an increasing gradient adjacent to the core's outer surface; and forming a cladding layer around the core, wherein the cladding layer is doped with fluorine, and wherein the concentration of the fluorine dopant has a decreasing gradient from a maximum value adjacent to the core.

9. A process as defined in claim 8, wherein:

the cladding layer formed in the step of forming a cladding layer is an interface cladding layer; the process further comprises forming an outer cladding layer over the interface cladding layer; and the outer cladding layer comprises substantially pure silica glass.

10. A process as defined in claim 8, wherein the increasing gradient in the concentration of the germanium dopant in the core, and the decreasing gradient in the concentration of the fluorine dopant in the interface cladding layer, are selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the optical fiber preform is subsequently being processed.

11. A cylindrical optical fiber preform comprising:

a core doped with a germanium dopant having a concentration that is substantially uniform throughout the core, but that increases immediately adjacent the core's outer surface; and a cladding layer disposed over the core and doped with a fluorine dopant having a concentration that decreases from a maximum value immediately adjacent to the core.

12. A cylindrical optical fiber preform as defined m claim 11 , wherem:

the cladding layer includes an interface cladding layer disposed over the core and an outer cladding layer disposed over the interface cladding layer; and the outer cladding layer comprises substantially pure silica glass.

13. A cylindrical optical fiber preform as defined in claim 11 , wherein the increase in the concentration of the germanium dopant in the core and the decrease in the concentration of the fluorine dopant in the cladding layer are selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the optical fiber preform is subsequently being processed.

14. A cylindrical optical fiber preform made using the process of claim 1.

15. A cylindrical optical fiber preform comprising:

an inner core formed using a vapor axial deposition apparatus, wherem the inner core is doped with a germanium dopant having a prescribed, substantially uniform concentration; an outer core disposed over the inner core and formed using an outside vapor deposition apparatus, wherein the outer core is doped with a germanium dopant having a concentration that increases from the prescribed, substantially uniform concentration adjacent the outer surface of the outer core; and a cladding layer disposed over the outer core and formed using the outside vapor deposition apparatus.

16. A cylindrical optical fiber preform as defined in claim 15, wherein the cladding layer is doped with a fluorine dopant having a concentration that decreases from a maximum value immediately adjacent to the outer core.

17. A cylindrical optical fiber preform as defined in claim 16, wherein:

the cladding layer disposed over the outer core constitutes an interface cladding layer; and the preform further comprises an outer cladding layer formed of substantially pure silica glass.