Patent Application
20090193851
1. Field of the Invention
The present invention generally relates to optical fibers and, more particularly, to preforms for the fabrication of optical fibers including highly non-linear non-conventional glasses as a core material thereof.
2. Description of the Prior Art
Fiber optics are becoming of very widespread use for communication links due to their degree of noise immunity and potential bandwidth. Numerous designs of low loss optical fibers are known and commercially available in long lengths and their performance is well-documented and subject to industry standards. Fiber optics also have many applications such as in light amplifiers where nonlinear effects are of primary importance and require non-standard fibers with very high non-linearity in short lengths. Such fibers with very high non-linearity glasses in the core are known as highly non-linear fibers (HNLFs). Additionally, optical fibers are often made in short lengths of highly doped glasses which are also drawn from preforms. An example of such glasses is MM2 glass from Kigre, Inc. which is highly doped with erbium but not a non-linear glass.
Attempts to fabricate HNLFs fall into three general categories which can be classified and are generally referred to as Modified Chemical Vapor Deposition MCVD techniques, crucible methods and rod-in-tube, respectively. MCVD techniques which involve a layer-by-layer deposition of material of a core (which can form a highly non-linear optical region) inside a cladding tube, offer the greatest potential for high purity and tight confinement but the processes are very slow and somewhat complicated, particularly for multi-component glasses in the core. (Cladding is a layer of material surrounding the core generally to increase efficiency of light transmission and reduce light loss. Such a layer surrounding the core may or may not function as cladding but the term “cladding” will be used herein as a collective reference to materials surrounding an optical fiber core, regardless of its actual function.) Crucible and rod-in-tube approaches usually involve processing of bulk glass samples or glass powders and using the cladding tube essentially as a crucible for the core glass where softening or melting of the core glass occurs principally during drawing of the optical fiber from the preform. Core glasses can be processed separately and then combined with cladding material using all of the above-mentioned techniques. Also, since HNLFs and highly doped glasses are non-standard, it may be desirable to include any of a wide variety of materials in the core glass, some of which may be highly toxic. None of the above known techniques of fabricating optical fiber preforms lend themselves particularly well to confinement of substances and vapors which may be toxic without the addition of particular structures for that purpose or other significant complication of the respective processes.
The most significant difficulty with these techniques derives from the desired small core diameter of the final fiber drawn from the preform. Non-linear glasses are usually associated with very high refractive indices as compared to silica. High refractive indices lead to a need for a very small core diameter between 0.5 and 1.0 microns to maintain the fiber single mode. A small core diameter increases the non-linearity by reducing effective area. Thus, most such HNLFs are fabricated by drawing fibers to the final desired size from preforms which can be substantially larger in cross-sectional dimensions. Even so, a 0.5 micron core in a 125 micron fiber requires that the inner diameter of a 1 cm cladding tube be only 40 microns. Thus, the core glass fiber would have to be drawn to a diameter of less than 40 microns prior to being inserted into a cladding tube (of a type which allows splicing to other fibers) before the assembly is drawn to final desired dimensions thus complicating and introducing additional failure modes into the assembly and drawing processes to produce the preform and then the optical fiber.
Further, additional problems have been encountered in known assembly and fabrication techniques for HNLFs. For example, trapped air entrained due to the necessary initial clearance between cladding and core may form bubbles within the cladding and it is anticipated that such bubbles will become more of a problem as the core and cladding bore become smaller. There is also a limit to the size from which a fiber having the desired final dimensions can be drawn due to the large thermal expansion coefficient (CTE) of high non-linearity glasses. These assembly and drawing techniques can also be sources and opportunities for contamination of the fiber at several stages of these processes.
It is therefore an object of the present invention to provide a technique of forming a preform for a highly non-linear optical fiber (HNLF) in which core material may be made to completely fill an inner bore of cladding material or the like and which can be spliced to other optical fibers.
It is another object of the present invention to provide an alternative, rapid, repeatable, robust and economical technique of producing optical fiber preforms to known techniques of fabricating optical fiber preforms, particularly for HNLFs.
It is a further object of the invention to provide an arrangement for fabrication of fiber optic preforms which allows safe fabrication even if toxic substances are to be included in the glass core.
It is yet another object of the invention to provide a technique of forming preforms, particularly preforms with very small diameter cores, and which will decrease the incidence of breakage and defects.
In order to accomplish these and other objects of the invention, a method is provided for fabricating optical fiber preform or an optical fiber comprising steps of melting or softening a glass material in a crucible enclosed in a furnace, introducing one end of a cladding tube into said furnace through an aperture in said furnace and below a surface of said glass material, and applying suction to an opposite end of said cladding tube to draw said glass material into said cladding tube.
In accordance with another aspect of the invention, an apparatus is provided comprising, in combination, a furnace including a heat source and a crucible containing molten glass, an arrangement such as an aperture for introducing a portion of a cladding tube into said furnace such that a first end of said cladding tube reaches a surface of said molten glass in said crucible, and a vacuum pump arrangement for applying vacuum to a second end of said cladding tube to draw said molten glass into said cladding tube by suction.
The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:
Referring now to the drawings, and more particularly to
It should be appreciated that the technique of drawing core glass into the bore of a cladding tube using suction is possible due to the fact that the melting or softening temperatures of highly non-linear core glasses and highly doped glasses are much less than that of the cladding tube which is generally of silica but lower melting or softening temperature glasses such as boro-silicate glass can also be used for lower softening or melting temperature core glasses. By the same token, the invention is completely applicable to fabrication of preforms for optical fibers of conventional composition as long as a suitable differential of melting/softening temperatures between the core glass material and the cladding material can be provided. Therefore, the bore of the cladding tube can effectively be used in the manner of a mold for the core glass which is drawn into it, thus eliminating voids and ambient gas entrainment and core rod breakage which have been observed when a solid core rod is inserted into the cladding tube. Furnace 10, when in use, has only one opening for the insertion of the cladding tube (and vacuum connection and thus is able to confine and toxic gasses which may be present or evolved from the molten glass. Such gasses which are drawn into vacuum pump 50 may be suitably confined or exhausted. Further, since the core glass is drawn into the cladding tube in a molten state, it is believed that incidence of breakage due to differences of the coefficient of thermal expansion between the cladding and core during drawing of the preform into an optical fiber at an elevated temperature is reduced.
To fabricate a preform using apparatus of the general type and having the features illustrated in
It will be recognized by those skilled in the art that the above process involves numerous interrelated parameters and many possibilities for process variation; each of which may be chosen within a relatively wide range consistent with successful practice of the invention. For example, the vacuum can be applied either before or after the insertion into the muffle furnace and crucible. The choice may involve such factors as the desired temperature of the cladding tube and its specific heat which should be such that the molten core glass does not significantly solidify due to temperature drop through heat transfer to the cladding tube during the suction process and drawing the high temperature ambient atmosphere within the furnace through the cladding tube prior to suctioning of the glass may allow the cladding tube to reach the desired temperature more quickly.
In general, in this regard, it is preferable that the dimensions of the muffle furnace 10 be such that substantially all of the cladding tube can be received therein with no more extension, if any, outside the muffle furnace than is necessary for connection to the vacuum pump 50. For longer preforms, a so-called horizontal tube furnace can be oriented vertically to accommodate the cladding tube vertically therein and used in place of a muffle furnace. Elongated furnaces such as those conventionally used for fiber drawing operations can also be used as well as furnaces of custom dimensions to accommodate preforms of whatever dimensions may be desired and can include such features as silica liners and structures 70 such as for passing inert gases or other controlled atmospheres over the cladding to further reduce the potential for contamination can be provided as schematically and collectively illustrated in
Similarly, the level of vacuum applied is generally not critical to the practice of the invention but should be great enough to complete the suction process in a reasonable time (and with good uniformity if diffusion from the core to the cladding may be significant) based on the viscosity of the molten core glass but low enough to avoid any out-gassing from the molten glass other than above the molten glass column or breakage of portions which are allowed to solidify during the suction process (e.g. using reduced cladding tube preheating). The temperature of the molten core glass will affect viscosity of the core glass which should be low enough that molten core glass can continuously flow to the cladding tube bore during the suction process without cavitation or aspiration of furnace gasses into the cladding tube bore as well as flowing within the usually very narrow bore of the cladding tube with an acceptable and practical level of vacuum.
These and other concerns regarding the interrelationships of variable process parameters will be apparent to those skilled in the art in order to predict combinations of process parameters which might produce flaws in the preforms. However, it is to be understood that the process is extremely robust, empirical in nature and may be successfully practiced over a wide range of values of process parameters and variations. Even the first experimental trial of the method described above using a mixture of pure GeO2 and TeO2 powders was successful in fabricating an HNLF optical fiber preform. Therefore, the discussion of variation in the parameters of the process in accordance with the invention will principally assist in avoiding combinations of parameter values which can be projected to be unsuccessful.
The method has also been successfully and repeatedly performed with good yield using conventional materials such as Schott SFL6 glass to form preforms with core diameters ranging from 22 to 45 microns from which optical fibers with suitably small core diameters, as noted above, have been successfully drawn with good yield. Other core glass materials successfully used in accordance with the invention to date also include Lead-Germanate-Telluite based materials of various compositions and many other glass formulations with which the invention has provided uniformly successful results. It should also be appreciated that layers of other materials such as a barrier layer may be applied within the cladding tube (e.g. by MCVD) for any desired purpose or function.
In view of the foregoing, it is clearly seen that the invention provides an alternative technique for forming small diameter preforms from which very small diameter optical fibers of relatively short length can be drawn, even when using core materials suitable for HNLF fibers. The method in accordance with the invention is a rapid, repeatable, robust and economical technique of producing optical fiber preforms and uses a furnace which is preferably closed and/or amenable to providing a controlled atmosphere therein and thus can also confine materials which may be toxic and controlling sources of potential contamination which, in any event, are much reduced in number and likelihood of causing contamination than the greater number of steps of known processes for making preforms. Potential sources of other defects such as entrained air are also substantially avoided in accordance with the invention and toxic materials, if present may be readily confined.
The technique in accordance with the invention is principally suitable for but not limited to, highly doped non-conventional multi-component glasses. These novel glasses, mainly used for optical amplification (such as Raman amplifiers and EDFA amplifiers), lasers, lidar, etc. applications, could either be tellurium, bismuth, germanium of the like based highly non-linear glasses or highly doped rare earth glasses like erbium or neodymium or the like or any other material of combination thereof.
While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.
This application claims benefit of priority of U.S. Provisional Patent Application Ser. No. 60/680,045, filed May 12, 2005, which is hereby incorporated by reference in its entirety.
The development of this invention was funded by the National Science Foundation, grant number ECS-0123484 and the National Institute of Aerospace, grant number VT-03-1. Accordingly, the United States Government has certain interests in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60680045 | May 2005 | US |
