Patent Application
20130025326
The present invention relates generally to the field of optical wave guide fibers, and more particularly to optical waveguide preforms and methods of making optical waveguide preforms, from which low water peak optical waveguide fibers are manufactured.
A significant goal of the telecommunications industry is to transmit greater amounts of information over longer distances, in shorter periods of time. Over time there has also typically been an increase in the usage of telecommunication systems, by users and by system resources. This has resulted in demands for increased bandwidth in the media used to carry this information over long distances, in particular for optical waveguide fibers that are contained in telecommunication cables.
Bandwidth in optical waveguide fibers is dependent on a number of factors, such as the attenuation of the fiber at the transmission wavelength. Impurities present in the light guiding region of the fiber can increase the attenuation of the fiber, due to absorption of the transmitted light. Of particular significance is the attenuation caused by the hydroxyl radical (OH), which can be bonded to the fiber structure during the manufacturing process. The presence of OH bonds in the light guiding region of the fiber can cause an attenuation increase, with a peak of attenuation in a window around 1380 nm, also generally referred to as the water peak. The 1380 nm window is generally defined as the range of wavelengths between about 1330 nm to about 1470 nm, with the peak attenuation effect typically around 1383 nm.
The present invention relates to methods of manufacturing a large low water glass optical waveguide core preform at high production rates. The glass optical waveguide preform is used to manufacture low water-peak optical waveguide fiber. The manufacture of doped silica products is described. All the processes described herein are equally applicable to the manufacture of non-doped silica products in the case where silica-based reaction products contain no dopants.
One aspect relates to a method of fabricating a porous core body which comprises steps of chemically reacting at least some of the constituents of a moving fluid mixture with at least one glass forming precursor compound in an oxidizing medium to form a silica-based reaction product. At least a portion of the reaction product, which contains hydrogen bonded to oxygen, is collected or deposited to form a silica-based porous core body, which preferably comprises a dopant such as germanium dioxide. The porous core body thus formed is typically subjected to a heat treatment in a furnace, during which a gas mixture may be passed through the furnace, which dries and compacts the porous core body.
In another aspect, two coatings of silica-based soot are deposited on a bait rod, the first of which contains a dopant and the second of which does not contain a dopant, forming a porous core preform. The porous core preform is then chemically dried and sintered to form a glass core preform.
In another aspect, a method of manufacturing a glass core preform includes several steps. First, a silica-based core material is deposited on a target of deposition comprising a rotating bait rod to form a preferably substantially solid soot cylindrical initial core preform. The initial core preform is preferably held at one end by the bait rod and is free at an opposing end. Then, additional silica-based core material is deposited on the target via a reciprocating (e.g., back and forth along the longitudinal length of the target) deposition. Cladding material is then deposited on the target via a reciprocating deposition to form a final core preform. Then, at least a portion of the final core preform is dried and sintered to form a glass core preform.
Preferably, a handle is positioned against or close to the opposing end of the initial core preform after the initial axial deposition and before the reciprocating depositions. The handle and the rod at the opposite end of the preform is then treated as part of the target for the subsequent reciprocating depositions.
In another aspect, a method of manufacturing a glass core preform includes several steps. First, a silica-based core material is deposited on a target of deposition comprising a rotating bait rod to form a preferably substantially cylindrical solid soot initial core preform. The initial core preform is preferably held at one end by the bait rod and is free at an opposing end. Simultaneously with the formation of the cylindrical solid initial core preform additional silica-based core material is deposited on the target. If needed, additional silica-based core material is deposited on the target via a reciprocating deposition. Cladding material is then deposited on the target via a reciprocating deposition to form a final soot core preform. Then, at least a portion of the final soot core preform is dried and sintered to form a glass core preform.
In another aspect, a method of manufacturing a glass core preform includes several steps. A silica-based material is deposited on a target of deposition comprising a rotating bait rod to form a substantially cylindrical initial core preform. The initial core preform is held at one end by the bait rod and is free at an opposing end. Cladding material is then deposited on the target via a reciprocating deposition to form a final core preform. Then, at least a portion of the final core preform is dried and sintered to form a glass core preform. The drying and sintering steps are performed under conditions suitable to make an optical fiber having an attenuation of less than about 0.35 dB/km, and preferably less than about 0.31 dB/km, at a wavelength of 1380 nm.
In another aspect, the glass core preform is drawn into glass core rods, which function as a substrate for the further deposition of cladding silica soot by an OVD method to form a porous optical waveguide preform. The porous optical waveguide preform is chemically dried and sintered, to form a glass optical waveguide preform, so that the optical waveguide fiber producible from these preforms exhibits an optical attenuation of less than about 0.35 dB/km, and preferably less than about 0.31 dB/km, at a measured wavelength of about 1380 nm.
In another aspect, the glass core preform comprises a doped centerline region of such dimensions that it is suitable for forming a glass optical waveguide preform that can be drawn into optical waveguide fiber, where the fiber producible from these preforms exhibits an optical attenuation of less than about 0.35 dB/km, and preferably less than about 0.31 dB/km, at a measured wavelength of about 1380 nm.
The methods disclosed herein result in a number of advantages over other methods known in the art, including the following:
1. The traditional OVD method of core preform production requires the use of a removable substrate which forms a centerline hole; this hole remains in the glass core preform after drying and sintering. The water peak is largely a result of water being trapped in the glass during the fiber manufacturing process, and in the case of the OVD process a large portion of the water is trapped in the centerline hole region prior to the hole being closed. The most common cause of the water being trapped in the centerline hole is through rewetting of the glass by exposure to an atmosphere that contains a hydrogen containing compound, such as, but not limited to, water. The present method produces a core preform with no centerline hole in the core region, eliminating the rewetting mechanism.
2. The traditional OVD method of core rod production closes the centerline hole in the core preform by applying a vacuum along the centerline hole during the core rod drawing process. The conventional method can cause core rod losses, due to the formation of voids or bubbles along the centerline during incomplete hole closure. Additionally, the hole closure process may be non-circular, potentially causing issues with fiber properties. The present method does not have these issues.
3. The conventional closure of the core preform centerline hole usually requires the use of hollow silica handles, ground glass joints, vacuum pumps and associated pipework. In the present invention, as there is no hole to collapse, the costs and associated difficulties with hole closure are eliminated.
4. The closure of the centerline hole typically creates a dip in the refractive index profile of the core rod. The methods disclosed herein result in more uniform refractive index profiles, as the dip in the center of the refractive index profile may be eliminated.
5. In one aspect, the glass preform resulting from sequentially performing VAD core deposition, OVD core deposition, OVD cladding deposition steps can be drawn directly into fiber without the additional stages of drawing into rods and further overcladding. This process eliminates a range of processing steps, and reduces manufacturing costs accordingly.
The disclosed methods combines the advantages of making core preforms via VAD in the centerline region with the advantages of OVD in the non-centerline regions. These advantages include high deposition rates, preform stability, large preform size, and high deposition efficiency outside the centerline region of the preform. Also, the porous core preforms are porous, with no centerline hole, allowing the core preforms to be thoroughly chemically dried, with no problem of rewetting within the centerline region of the core preform. Accordingly, the optical waveguide fiber made from the optical waveguide preforms exhibit a much smaller water peak at 1380 nm, and exhibit a much lower attenuation in the window around 1380 nm, which is typical for VAD core preforms, than optical waveguide fiber manufactured in accordance with the standard OVD methods.
An additional advantage is that the optical waveguide fiber manufactured from optical waveguide preforms of the current invention can operate at any selected wavelength over a range of wavelengths from about 1300 nm to about 1680 nm without undue optical attenuation.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
Telecommunications systems traditionally have avoided using the water peak region, partly due to the lack of optical waveguide fiber with low water peaks. Fiber manufacturers now produce low water peak fibers by various methods. The development of methods for producing low water peak fibers has coincided with the development of telecommunication systems that increasingly use all the wavelengths between about 1300 nm and 1650 nm. For telecommunication systems to fully utilize this wavelength range, removal of the water peak from the optical waveguide fiber is required.
There are three main methods of optical waveguide preform manufacture in common use. The three techniques have similar methods of vapor generation and oxidation, but differ in the geometry of the substrate on which the oxide soot is deposited:
(i) Deposition in Tube Methods
These methods comprise techniques known as MCVD (Modified Chemical Vapor Deposition) and PCVD (Plasma Chemical Vapor Deposition). In these techniques, a vapor stream is introduced to the end of a high-purity quartz tube, and the oxides are deposited on the inner surface of the tube.
(ii) VAD (Vapor Axial Deposition)
In this technique, the deposition takes place on a usually vertically mounted rotating mandrel, and the preform is “grown” axially from a short stub into a longer, cylindrical preform. Methods for manufacturing a preform using a rotating mandrel and growing a preform axially are described in U.S. Pat. No. 5,583,693, issued to Sarkar, which is incorporated by reference as though fully set forth herein.
(iii) OVD (Outside Vapor Deposition)
In this technique, silica-based soot is deposited on a rotating target rod. The rod builds up to form a cylindrical soot preform, which can be sintered and dried to form a glass preform. For example, U.S. Pat. No. 6,477,305, which is incorporated by reference as though fully set for the herein, discloses a method of eliminating the water content in the doped core portion of preforms caused by preforms' centerline hole following removal of the target rod.
The methods and embodiments of the present invention are particularly applicable to optical waveguide preforms manufactured using the VAD process.
The VAD process, while offering a low water peak solution for manufacturing optical waveguide fiber, is limited in its utility because of the size of the core preforms it can produce. The doped core portions of such preforms are limited in size to what can be deposited in one pass of a deposition. The VAD core preform sizes are also limited by the weight that the deposited soot can support before breaking. Modern preforms need to be large to keep down the cost per kilometer of fiber. Core preforms are sought having a minimum central core doped mass of 800 grams so that an at least a 10 kilogram core preform can be manufactured. Using the VAD method, the core preform would break long before it is completed, where such preform has an approximately 800 gram doped central core.
Moreover, VAD deposition rates are relatively very low. One reason for this is that the targets are never large enough to capture large amounts of soot. Another reason for the slow deposition rates in VAD is the relatively low thermophoretic force between the soot and the developing preform. The slow translational speed of the deposition in VAD results in a lower thermophoretic force, and as a result, a lower deposition efficiency. The slow speed also results in more tapered preforms that have large unusable portions as discussed in U.S. Pat. No. 6,789,401. In VAD, increasing the translational speed of the deposition to eliminate these problems typically breaks soot preform because of the accelerating and decelerating forces that accompany higher speeds. A need therefore exists to generate large preforms with a low water peak that overcomes these issues.
Commercial OVD core manufacturing equipment and processes are available that simultaneously manufacture two 11 kilogram core preforms having an 800 gram central core doped mass at deposition rates of around 12 grams per minute per preform. Such an OVD machine and process produces sufficient core preforms to make the cores for five million kilometers of fiber per year. Producing VAD cores at rates similar to OVD core production is also needed. The systems and methods disclosed herein solves the aforementioned problems.
A first chemical source 103, such as a vaporizer, contains SiO2 (silicon dioxide) precursor materials, such as SiCl4 (silicon tetrachloride). A second chemical source 104 is provided, such as a vaporizer containing precursor dopant materials, such as GeCl4 (germanium tetrachloride). SiCl4 and GeCl4 vapors exit the SiCl4 source and GeCl4 source 103, 104, typically. In an optional embodiment, the precursor SiO2 material may be OMCTS (octamethylcyclotetrasiloxane). The system 100 includes a gas carrier line 107 that is configured to provide fuel, such as H2 or natural gas and O2, from a source 111. Valve 106 may be opened or closed to allow or shut off the GeCl4 source 104 when desired, such as when cladding deposition proceeds. The glass forming compounds are mixed and dissociated, as is well known in the art, upon being provided to a first burner 108. In
Silica bait rod 120 is positioned along the reference axis on chuck 122. The chuck 122 and bait rod 120 are rotated by a rotary drive 124 mounted on a linear traverse mechanism 126. A position controller 128 runs traverse mechanism 126 at a desired rate.
A second burner 130 is fed deposition material using the first chemical (e.g., SiCl4) source 103 when valve 132 is open. Burner 130 optionally receives material from the second chemical source (e.g., GeCl4) when the valve 106 is also open. A second soot stream 134 is generated that is preferably aimed substantially perpendicular to the longitudinal axis of the preform. A torch 109, which is also provided with fuel such as H2 or natural gas and O2 from the source 111, is also preferably positioned between the burners 108, 130 to help control the density of the deposition. Throughout the deposition process, torch 109 preferably maintains a fixed lateral position relative to the burners 108 and 130. Additional torches may be added to support this purpose at different locations along the length and/or radial position of the developing substantially cylindrical preform.
Formation of a core preform using VAD approaches as are well know in the art may be performed using deposition chamber 101. Examples of suitable VAD approaches are disclosed in U.S. Pat. No. 5,558,693 issued to Sarkar. A starter tip 136 develops over the free end of the bait rod 120. When sufficient material has been deposited, the tip 136 forms an adequate base for the manufacture of a solid soot core cylinder 138.
Handle 140 is preferably positioned along the reference axis on chuck 142, to be in proximity or alternatively in contact with the free end of the solid soot core cylinder 139 once the cylinder 139 achieves a predetermined length. Solid soot core cylinder 139 is essentially solid soot core cylinder 138 after it has achieved the desired length. Handle 140 is preferably concave and matched to the convex curvature of the free end of the grown initial core preform 139. Because of the potential for air gaps between the handle 140 and the preform 139, handle 140 preferably includes a hole through the ends of handle 140. The handle 140 and second chuck 142 are rotated by a second rotary drive 144 synchronized to the rotation of drive 124. Rotary drive 144 is optionally mounted on linear traverse mechanism 146 that is run by position controller 128 or a second position controller 148 synchronized with position controller 128, as needed.
End torches 150 and 152 are also preferably provided in the deposition chamber 101. The end torches 150, 152 are preferably connected to the respective traverse mechanisms 126, 146. The end torches 150, 152 also receive fuel from the fuel (e.g., H2 or natural gas and O2) source 111. The end-torches 150, 152 are preferably active during the deposition process to keep the ends of the preform hot. Doing so prevents the preform from cracking, a phenomenon that typically starts at the ends of the preform where thermal expansion and density coefficient mismatches are most severe.
A solid cylindrical glass optical waveguide preform 102 from which optical waveguide fiber is manufactured comprises a central core region comprising silica material, such as for example, glass SiO2, combined with a dopant (preferably GeO2 (germanium dioxide)), surrounded by a cladding region comprising silica material, such as for example, glass SiO2. The core region preferably extends longitudinally along the central axis of the cylindrical optical waveguide preform.
In one embodiment, the porous core body is formed by chemically reacting at least some of the constituents of a moving fluid mixture comprising at least one glass-forming precursor compound in an oxidizing medium. The reaction results in the formation of a silica-based reaction product (soot) which can be doped or undoped. At least a portion of this reaction product is directed toward a bait rod, to grow the porous body. The porous body may be formed, for example, by depositing silica-based reaction product on the free end of the axially growing preform, such as via a VAD process as is known in the art.
In so doing, an initial core preform is grown from the bait rod. Using the deposition system of
Once the predetermined core soot preform length is achieved, in a next step 204, a handle is preferably positioned proximate to the free end of the preform so as to bear any stresses that may cause the preform to break. The handle, preferably matched to the curvature of the end of the axially grown preform, preferably rotates at the same speed and direction as the core soot preform and remains consistently proximate to the free end of the preform.
In a next step 206, additional core material is deposited on the target. The deposition, however, rather than the axial deposition performed in the previous step by the burner 108, is radial, using one or more burners that are preferably depositing on a reciprocating target consistent with OVD techniques that are well know in the art. Employing, for example, the deposition system of
While a two-burner configuration as depicted in
Preferably, the target for the reciprocating deposition includes the core preform 102 and 138, the handle 140 and the bait rod 120. Depositing on the handle and bait rod allows for the handle and bait rod to support the weight of the preform as its mass increases. The attachment of and deposition on handles in the manufacture of low water-peak optical fiber preforms is described in U.S. Pat. No. 7,930,905, which is incorporated by reference as though fully set forth herein.
One advantage of employing the OVD process at this stage is that the OVD process provides for deposition of layers that are longitudinal with respect to the preform as opposed radial, which is what is produced by the VAD deposition method employed in step 202. The longitudinal deposition layering provides for a stronger preform that is more resistant to breaking as the mass of the preform increases with further deposition of soot. This may be particularly significant where the OVD process is being performed horizontally, where gravity may tend cause cracks or breaks of the preform as its mass increases with the weight of the preform being born by the handles.
Employing a handle, however, is also advantageous where the OVD process is performed vertically. Including a handle on the free lower end of the core preform before the OVD process commences makes the preform more stable, and allows the preform to handle the stresses of a high-speed OVD process and to avoid breakages in the developing preform. Methods and advantages of performing OVD with high-speed passes are well-known in the art, such as are described in U.S. Pat. No. 6,789,401.
In the OVD process, the bait rod is preferably mounted on a lathe, which is designed to translate and rotate the bait rod, in close proximity to a soot-producing burner, such as burner 130 of
Once the doped core soot preform has achieved a predetermined mass, such as at least about 400 grams, and preferably about 800 grams, the next step 208 is performed, in which cladding material, preferably in the precursor form of SiCl4 or OMCTS, is deposited on the target. Again, the target of the deposition preferably includes the preform and the handle and bait rod at respective ends of the preform. Again this phase of the deposition is performed via reciprocating preform or burners, such as burners 108 and 130 of
Once the targeted quantity of soot has been deposited, the soot deposition preferably is terminated. In a next step 210, the core soot preform is dried and sintered using techniques as are known in the art to consolidate the soot material. Preferably, the porous core preform is positioned and rotated in a furnace for heat treatment. The porous core body is preferably subjected to a temperature of about 1100 to 1250° C. while still retaining its porosity. During this heat treatment process, the porous core preform is preferably chemically dried by exposing the body to a chlorine-containing atmosphere, which effectively removes water and other impurities from the preform. The soot preform is then sintered to glass at a temperature about 1500° C. in a helium atmosphere that may contain some chlorine to avoid rewetting. The result of this process is a glass core preform, having a density preferably of about 2.2 g/cm3, which through yet additional processing may be drawn into optical fiber for telecommunications applications.
In accordance with one preferred embodiment, the porous core preform is positioned within a sintering furnace and rotated, where the preform is chemically dried at a temperature of preferably about 1100° C. in an atmosphere of chlorine and helium. Following drying, the porous core preform preferably is driven down into the hot zone of the sintering furnace preferably in an inert gas atmosphere, such as helium, and then sintered at an elevated temperature, preferably at about 1500° C., to thereby form a glass core preform.
In one embodiment, the glass core preform is taken to a core rod drawing furnace, where the preform is drawn into a number of reduced diameter core rods. As the glass core preform is solid glass, without a centerline hole, there is no need for the application of vacuum to the preform during core rod drawing, as there is no possibility of the centerline region being rewet by exposure to the ambient atmosphere and there is no need to close a hole. Deposition by the OVD method continues on the glass core rods or the original glass core preforms to produce glass optical waveguide preforms, which are then drawn to optical wave guide fiber.
In another embodiment, after deposition has been terminated, the dimensions of the core region and the cladding region are such that the dried and sintered preform is a glass optical waveguide preform. This glass optical waveguide preform may be directly drawn to optical waveguide fiber. The resulting optical waveguide fiber produced by this method has an attenuation of no greater than 0.35 dB/km for light at a measured wavelength 1380 nm, and preferably less than 0.31 dB/km.
Numerous references describe the manufacture of preforms by overcladding core rods by OVD soot deposition, such as described in U.S. Pat. No. 7,930,905, or by performing rod-in-cylinder processes, such as described in U.S. Pat. No. 6,131,415.
The foregoing detailed description of our invention and of preferred embodiments as to products, compositions, and processes is illustrative of specific embodiments only. It is to be understood, however, that additional embodiments described herein, together with those additional embodiments, are considered to be within the scope of the present invention.
