METHOD FOR CREATING CODOPED LAYERS AND FIBERS CONTAINING CODOPED LAYERS

Abstract: A method of creating a codoped layer (18) includes creating a first layer (14) having a first dopant and at least one other layer (16) have another dopant, then interdiffusing the dopant to create a substantially homogeneous codoped layer. More than one dopant may be deposited in a single layer. The creating conditions may be optimized for each layer (14, 16). Further, when the creation of a layer includes sequential deposition and consolidation, conditions for each process may be optimized within the layer creation. While at least two layers (14, 16) are formed, the interdiffusion substantially eliminates any stratification or layer structure.

Description: METHOD FOR CREATING CODOPED LAYERS AND FIBERS CONTAINING CODOPED LAYERS BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to a method of creating codoped layers, particularly codoped layers in fibers, more particularly codoped layers having high dopant concentrations serving as cores in fibers. Description of Related Art A photosensitive optical device is a device whose refractive index may be altered by exposing the device to optical radiation, typically in the ultraviolet region of the spectrum. Photosensitive optical devices have been known for a number of years. Currently, there is a great deal of interest in photosensitive devices, particularly in photosensitive fiber gratings, due to their ease of fabrication and their use in a wide variety of applications, particularly in the fields of telecommunications and sensing. Large photosensitivity is desired for allowing the creation of large photo induced index changes. Improved photosensitivity allows numerous new applications, such as very broadband reflectors /filters, ultra-short gratings, cladding mode suppression gratings, and other photonic bandgap devices, to be realized. It is known that codoping fibers, e.g., with germanium and boron or germanium and tin, enhances the photosensitivity of the best photosensitive single dopant fiber, i.e., doped with germanium. Prior to the increased requirements placed on photosensitive fibers, high concentrations of codopants in the core were typically not needed. Of course, codoped fibers may be employed for other uses and the problems with achieving codoped fibers with high levels of dopant concentrations are the same regardless of end use of the fiber. As used herein, codoped refers to two or more types of dopants being present in the same region, as opposed to fibers such as gradient index fibers which may have different dopant concentrations in different stratified layers. In conventional codoping processes, all dopants are introduced into the process simultaneously to form a codoped fiber core. However, typically, different dopants require different, conditions, e.g., temperature, inside atmosphere, etc., for optimum collection efficiency. In the specific example of using B and Ge, B doping requires a relatively lower temperature than Ge dopant deposition. Further, BC1 3 oxidation generates locally high concentrated Cl environment, which dramatically reduces GeCl oxidation. Moreover, B O 3 decreases the glass viscosity, which in tun accelerates GeO thermal decomposition. In the worst case scenario, the Ge incorporation is prevented to such a degree that the core index change relative to the initial material is negative. SUMMARY OF THE INVENTION The present invention may be realized by a method of making a codoped layer including creating a first layer having a first dopant, creating a second layer having a second dopant over the first layer, the creating of the second layer being sequential to the creating of the first layer, and interdiffusing the first layer and the second layer to substantially eliminate layer structuretherebetween. The creating of each of the first and second layers may include depositing a dopant and consolidating the layer. The consolidating may include sintering the layer. The depositing and the consolidating may be performed sequentially. The consolidating may be performed under different conditions, e.g., temperature, atmosphere, than the depositing. Both the depositing and the consolidating may be performed under different conditions, e.g., temperature, atmosphere, for the first and second layers. Before interdiffusing, a third layer having a third dopant may be created over the second layer. The third dopant may be the same as one of the first and second dopants. Adjacent layers need not have different dopants. Layers may have more than one dopant present. Before either creating of the first layer and/or the second layer, a region on which a layer is to be created may be cooled. Additionally, the invention may be realized by a method of making a fiber having a codoped layer therein including creating a first layer having a first dopant in a fiber structure, creating a second layer having a second dopant over the first layer, the creating of the second layer being sequential to the creating of the first layer, interdiffusing the first layer and the second layer to substantially eliminate layer structure therebetween, and processing the fiber having the codoped layer therein to form a fiber having a desired dimensions. The method of creating of the first layer and the of the second layer may occur in a core region of the fiber structure. These and other aspects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which: Fig. 1 a shows an end face cross-section of a tube and cladding in which the core is to be formed; Fig. lb shows a longitudinal cross-section of the tube and cladding of Fig. la; Fig. lc shows a longitudinal cross-section of the tube and cladding of Fig. lb with a first layer deposited thereon-, Fig. Id shows a longitudinal cross-section of the structure of Fig. lc with a second layer deposited thereon; Fig. le shows a the structure of Fig. Id after collapsing; and Fig. 2 is a flow chart of the method in accordance with the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the present invention, during creation of the fiber, dopants are incorporated independently. This independent incorporation eliminates one feature of the modified chemical vapor deposition (MCVD) commonly employed to make fibers, e.g., a single pass creation of the core, including simultaneous deposition and consolidation. When doping the core with more than one dopant, this single pass creation is still used for all the dopants at the same time. Independent creation of layers in accordance with the present invention allows the creation conditions for each layer to be optimized, since conditions tailored for each dopant can be used. Then, interdiffusing of the created layers allows a substantially homogeneous dopant distribution to be achieved. The optimization of the collection of the dopants may be further improved by also sequentially performing the deposition and the consolidation involved in the creation of each layer. While sequential deposition and sintering have been employed to increase throughput of single dopant fibers or to create fibers with distinct dopant regions, such sequential deposition has not been used in conjunction with manufacturing codoped fibers having a continuous distribution of the dopants. In accordance with the present invention, sequential deposition and consolidating allows the collection efficiency of the dopants to be further optimized. The optimization of the collecting of the dopants may be realized since efficient deposition typically occurs at temperatures below a temperature required for consolidation. Further, in accordance with the present invention, the separation of the depositing and consolidating allows the consolidation environment to be altered from the typical oxidizing conditions to a condition which may further aid in enhancing photosensitivity. For example, when Ge is one of the dopants being incorporated, performing sintering in a reducing condition is believed to create more Ge related oxygen deficient centers, which increase the photosensitivity. Thus, the present invention may be used to create fibers having increased photosensitivity both by increasing the amount of dopants incorporated into the fiber and by altering the sintering environment to enhance photosensitivity. Fig. la shows an end cross-section and Fig. lb shows a longitudinal cross section of a tube 10 and a clad 12 in which the core material is to be deposited in accordance with the present invention. The creation of the structure in which the codoped layer is to be formed is conventional up to this point. It is noted that all of the structural figures are for illustration purposes only. They are not to scale and the thickness thereof, including relative thickness, may be exaggerated for clarity. Further, the longitudinal cross-sections sήown in Figs, lb-le are only partial cross sections, as indicated by the break lines at the ends thereof. Then, a first dopant is supplied under the optimum conditions for such deposition and consolidated under the optimum conditions to form a first layer as shown in Fig. lc. Then, another dopant is supplied under its optimum conditions for deposition and consolidated under the optimum conditions to form a second layer 16 as shown in Fig. Id. The deposition and consolidation of various dopants under their respective optimum conditions may be repeating any desired number of times. Further, to insure homogeneity of the final codoped layer and/or to provide sufficient deposition of the dopants, varying layers of the same dopants may be supplied, e.g., first a Ge layer, then a B layer, then another Ge layer, then another B layer, etc., or a plurality of adjacent layers containing the same dopant. Further, a layer may have more than one type of dopant therein. Additionally, adjacent layers do not have to have different dopants, although more than one dopant will be provided over the plurality of layers. As shown in Fig. le, once all the desired layers have been deposited and consolidated, the structure is then collapsed to form a preform having a homogeneous core 18, i.e., no layer structure remaining from the alternating deposition can be discerned. This preform is then ready to be drawn into a fiber of desired parameters or otherwise further processed to achieve desired core/clad ratios. A summary of the process of the present invention is illustrated in Fig. 2. The summary begins after the tube structure of Fig. la, or other appropriate structure in which the codoped layer is to be created, has been formed. In 20, the tube is allowed to cool down to increase soot collection efficiency and stabilize chemical reactant flows. Typically, the maximum temperature of the tube for the deposition is below 200°C. Then, in 22, a first dopant is deposited under desired conditions. Typically, the desired conditions will be conditions for obtaining maximum collection efficiency of the desired dopant. In 24, the layer is consolidated, e.g., by sintering, which may be performed under different conditions then the depositing. In 26, the tube with the first layer therein as shown in Fig. I c is allowed to cool if it is not already at the desired temperature. Then, in 28, another layer with another dopant is deposited under desired conditions. The other dopant will typically be different from the first dopant. Thus, the deposition conditions will typically be different from the previous conditions in 22, and will typically be the deposition conditions for realizing the maximum collection efficiency of the other dopant. The other layer is then consolidated in 30, e.g., by sintering, under conditions which may be different from the previous sintering in 24. As indicated in 31 , any additional number of layers of any desired combination, e.g., all of which may be different dopants, some of which may be the same dopant as previously deposited, some may contain more than one dopant, etc., may be deposited and consolidated under their respective desired conditions. Further, as noted above, while layers are to be sequentially created, the creation of each layer may be achieved in a single pass, rather in the sequential manner shown. In other words, 22, 24 may be simultaneous and 28, -30, while subsequent to 22, 24, may be simultaneous with each other. If all desired layers have been created, the flow proceeds to 32. In 32, once all of the desired layers have been deposited and consolidated, the entire structure is collapsed to form the preform shown in Fig. le. The collapsing promotes the interdiffusion of the dopants and homogenizes the composition across the core layers. The presence of layers in the core should not be discernible in the resulting structure in terms of the refractive index profile. This collapsing may be performed in a conventional manner, preferably at a temperature above the consolidating temperatures for the layers. The maximum thickness for a layer above which sufficient inter-diffusion will not result depends on the specific dopant diffusion coefficients. However, under most deposition processes, any resultant thickness will allow sufficient inter-diffusion by collapsing the preform. A specific example is discussed below. A standard clad containing Ge, F and P as well as Si is deposited. The O flow in the SiCl 4 bubbler is about 600 seem, 70 seem in the GeCl 4 bubbler, 75 seem in POCl 3 bubbler and 5 seem of SiF . Such a formation creates a barrier layer to prevent water diffusion from the Tube and the H /0 burner. F and P are introduced in order to reduce the processing temperature. Ge is introduced to compensate for the index decrease and to reduce draw induced attenuation. Before forming the core layers, the tube formed above is allowed to cool down to increase the soot collection efficiency and to stabilize chemical reactant flows for the following deposition. Core deposition begins with Ge deposition. The Ge soot layer is deposited at the preferred temperature, e.g.,1550°C. Then, this soot layer is consolidated by sintering. Since the deposition and consolidating are performed sequentially in accordance with this example, the conditions under which consolidating are performed may be different from those for the deposition to optimize the photosensitivity or other desired property of the resulting fiber without increasing dopant concentration. Such an increase may be realized, for example, when using Ge as one of the dopants, by consolidating including sintering using a reducing environment or an oxidizing environment. Preferably, this sintering is performed by flowing O and/or He and/or CO and/or Cl 2 and/or other gases depending on the environment requirement at, e.g., 1880°C. The next deposition is of B, and is actually a B/Ge codeposition. In this deposition pass, both Bcl 3 and GeCl 4 gases are flowed in. However, the processing conditions are optimized for B deposition. Therefore, the temperature is reduced to, e.g., 1450°C. Further, since the B layer has a lower melting point than that of the Ge layer, the consolidating includes sintering at a reduced temperature, e.g.,17000C. After B/Ge deposition and consolidation, another Ge layer is deposited, then another B/Ge layer, and then a final Ge layer for a total of five layers. The number of layers will depend on the desired end purpose and application. Creation of each layer typically preferably includes cooling, depositing and consolidating. Using this method, concentrations of B 2 -O 3 and GeO of greater than around 15 weight percent each can be realized. Once all of the desired layers are created, the tube is then collapsed into a preform. By using a high temperature in the collapsing function, i.e., above the consolidating temperatures, e.g., 2000-2100°C, interdiffusion of the dopants is promoted. This collapsing substantially homogenizes the composition of the core, i.e., eliminates the alternating layer structure. Therefore, in accordance with the present invention, a number of layers with different dopants can be separately created, preferably including separate deposition and consolidation for each layer, under optimized conditions. The resulting, structure may then be interdiffused to eliminate the layered structure. While the motivation for providing a method which allows an increased concentration of codopants is derived from the demands of photosensitive fiber gratings, the present invention may be employed to create a codoped fiber for any desired application. Further, while the above description is directed to forming a codoped core, the process of the present invention can be used to form any desired codoped layer. For example, in the fiber regime, ring structures, depressed claddings, photosensitive claddings, etc., all having more than one dopant and a substantially homogeneous refractive index profiled along the codoped region, may be created using the present invention. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. For example, while cooling prior to deposition is preferable, it is not necessarily required for all desired dopant levels. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

Claims: What is Claimed is: 1. A method of making a codoped layer comprising: creating a first layer having a first dopant; creating a second layer having a second dopant over the first layer, said creating of the second layer being sequential to said creating of the first layer; and interdiffusing the first layer and the second layer to substantially eliminate layer structure therebetween. 2. The method of claim 1, wherein said creating of the first layer comprises depositing the first dopant and consolidating the first layer. 3. The method of claim 2, wherein said consolidating the first layer includes sintering the first layer. 4. The method of claim 2, further comprising performing said consolidating under different conditions than said depositing. 5. The method of claim 2, wherein said creating of the second layer includes performing depositing under different conditions than for said depositing of the first layer. 6. The method of claim 2, wherein said creating of the second layer includes performing consolidating under different conditions than for said consolidating of the first layer. 7. The method of claim 1, wherein said creating of the second layer comprises depositing the second dopant and consolidating the second layer. 8. The method of claim 7, further comprises performing said consolidating under different conditions than said depositing. 9. The method of claim 1, wherein said creating of the first layer and said creating, of said second layer are performed under different conditions. 10. The method of claim 1, further comprising, prior to said interdiffusing, creating a third layer having, a third dopant over the second layer. 11. The method of claim 10, wherein the third dopant is the same as one of the first and second dopants. 12. The method of claim 1, wherein said interdiffusing includes collapsing the first and second layers. 13. The method of claim 1, wherein at least one of said first and second layers includes more than one dopant. 14. The method of claim 13, said more than one dopant includes a dopant which is the same as a dopant from the other of the first and second layers. 15. The method of claim 1, wherein one of the first and second dopants is Ge and another of the first and second dopants is B. 16. The method of claim 1, wherein said creating of each layer comprises sequentially depositing a dopant and consolidating the layer. 17. The method of claim 16, wherein said consolidating for at least one of the first and second layers comprises sintering under one of an oxidizing condition and a reducing condition. 18. The method of claim 1, further comprising, prior to at least one of said creating of the first layer and said creating of the second layer, cooling a region on which a layer is to be created. 19. A method of making a fiber having a codoped layer therein comprising: creating a first layer having a first dopant in a fiber structure; creating a second layer having a second dopant over the first layer, said creating of the second layer being sequential to said creating of the first layer; interdiffusing the first layer and the second layer to substantially eliminate layer structure therebetween; and processing the fiber having the codoped layer therein to form a fiber having a desired dimensions. 20. The method of claim 19, wherein said creating of the first layer and said creating of the second layer occurs in a core region of the fiber structure.

Inventors: QI GANG, SWAN MARTIN R, QI, GANG, SWAN, MARTIN, R.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2000
Main classification: G02B
IPC codes: C03B37, C03C13, G02B6