Abstract:
A filled-core optical fiber and method where the optical fiber is collapsed at opposing ends subsequent to the active optical material being introduced into the hollow core region. The collapsing-functions to “pinch off” the active material (which may be a liquid or solid) within the fiber structure and also collapse the cladding layer ring surrounding the core into a solid core region on either side of the active material. The filled-core fiber is then sealed and can be coupled to standard fiber using conventional splicing processes.

Description:
TECHNICAL FIELD 
     The present invention relates to a filled-core optical fiber structure and, more particularly, to a hollow core fiber filled with an optically active material and a method of making such a fiber so that it may be easily coupled to standard optical transmission fiber. 
     BACKGROUND OF THE INVENTION 
     Hollow-core optical fibers have become more prevalent in recent years as various uses for them have been developed. For example, a hollow-core optical fiber may be filled with a liquid crystal material and then used as an electrically-controllable long period fiber grating. See, for example,  Electrically Controllable Long - Period Liquid Crystal Fiber Gratings , by Y. Jeong et al., appearing in IEEE Photonics Technology Letters, Vol. 12, No. 5, May 2000, at pp. 519 et seq. Such a fiber has the same essential structure as a common transmission fiber, with the exception of the core region being filled to contain the desired active material. In the arrangement described by Jeong et al., a liquid crystal core fiber was formed by filling a hollow core fiber with a nematic liquid crystal material using capillary action. Index matching between a silica core transmission fiber and the liquid crystal-filled fiber was achieved by using a low index nematic liquid crystal material between the two. 
     In another utilization of a hollow-core fiber, a two-photon pumped laser has been formed by using a hollow-core fiber that is filled with a particular dye material that causes lasing or superradiance behavior. See, for example,  Two - photon - pumped cavity lasing in a dye - solution - filled hollow - fiber system ”, by G. S. He et al., appearing in Optical Letters, Vol. 20, No. 23, Dec. 1, 1995, at pp. 2393 et seq. In the particular lasing structure as discussed by He et al., the internal diameter of the hollow fiber was 100 μm and the two open ends of the fiber were immersed in two identical liquid coupling cells, each filled with the same dye solution as used in the core of the fiber. Each of the coupling cells further included an optical window to provide coupling out of the liquid-filled fiber and into the rest of the system. 
     Liquid-core optical fibers have also been used to measure temperature, as disclosed in U.S. Pat. No. 4,201,446 issued to Geddes et al. on May 6, 1980. In the Geddes et al. arrangement, a liquid-core fiber is disclosed in which the refractive index of the liquid core varies with temperature. Therefore, the temperature of the substance through which the liquid-core optical fiber passes can be measured from the maximum angle of the transmitted light output from the end of the fiber. The liquid-core fiber of Geddes et al. comprised a transparent capillary tube that was then filled with the temperature-sensitive liquid. The tube is then joined in series with conventional multimode fibers. There is no discussion in Geddes et al. regarding the degree of optical coupling that could be achieved with this approach, where the use of capillary tubes is known to cause reflections at the interface between the tube and the multimode fiber. 
     The ability to incorporate optically active materials (i.e., the optical properties of the materials can be altered by various means including the passage of an intense beam of light and the application of an external electric field) into hollow core fibers is of tremendous potential benefit. For instance, materials with high optical nonlinearities can be used to allow for more compact and lower power optical devices. It is asserted that the various prior art approaches to filling and sealing core-filled fibers are not practical for most optical system applications. In particular, the active core material is not sealed inside the fiber, which is a practical necessity for commercial device applications. Further, there is no known low-loss method for coupling light into and out of a core-filled optical fiber. 
     SUMMARY OF THE INVENTION 
     These and other problems remaining in the prior art are addressed by the present invention, which relates to a filled-core optical fiber structure and, more particularly to method of making such a structure that may be easily coupled to standard optical transmission fiber. 
     In accordance with the present invention, active core material is first introduced into a section of hollow core fiber, where the fiber is formed to comprise a high index cladding ring that surrounds the hollow core. The active core material may be, but is not limited to, a liquid that functions to alter the optical properties of a signal passing therethrough. Once the core is filled with a desired amount of active material, the end portions of the fiber (which do not contain the active material) are collapsed such that the high index cladding ring is compressed to form a high index core on either side of the core-filled fiber section, essentially “pinching” off and hermetically sealing the active material within the desired section of fiber. 
     In a preferred embodiment, the collapsing process is performed so that the final fiber structure includes opposing ends where the high index ring material forms high index core regions, followed by transition regions where the high index rings adiabatically taper outward to the central, core-filled fiber section. The adiabatic transition allows for low-loss mode evolution from the high index core region to the high index ring. 
     It is an aspect of the present invention that the collapsed fiber sections, including the high index core regions, can then easily be coupled (usually conventional techniques such as fusion splicing) to standard transmission fibers, thus forming a low-loss arrangement for coupling into and out of core-filled fibers. Moreover, the collapsed endpoints provide for a hermetic seal and ensure that the core material (in most cases, a liquid) remains in place within the core-filled fiber section. 
     Other and further aspects and benefits of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, where like numerals represent like parts in several views: 
     FIG. 1 contains an isometric view of an exemplary section of hollow-core optical fiber that may be filled with active material and sealed in accordance with the present invention; 
     FIG. 2 illustrates, in a cut-away side view, the hollow-core fiber section of FIG. 1, as it is filled with an active core material, such as a liquid or solid; 
     FIG. 3 illustrates the active core-filled fiber section of FIG. 2, subsequent to being collapsed to seal off the active core material and form high index end core regions; and 
     FIG. 4 illustrates the collapsed fiber section of FIG. 3 as it is attached to exemplary sections of conventional optical transmission fiber. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an exemplary section of hollow-core fiber  10  that may be processed to be filled with active core material and then sealed in accordance with the present invention. As shown, hollow-core fiber  10  comprises a high index cladding ring  12  that surrounds hollow core  14 , where an outer cladding layer  16  is formed to enclose high index cladding ring  12 . The outer cladding layer  16  has an index lower than that of the cladding ring  12 . For example, a typical diameter of the hollow core  14  is 5 μm, a typical width of the high index cladding ring  12  is 4 μm, and a typical index difference between the high index cladding ring  12  and the outer cladding layer  16  is approximately 0.005. Various conventional materials, such as germanium-doped silica and silica may be used to form high index cladding ring  12  and outer cladding layer  16 , respectively. It is to be understood that an exemplary hollow-core fiber may include other, similar geometries, and may include in particular additional cladding and outer protective layers. For the purposes of the present invention, the inclusion of hollow core  14  and high index cladding ring  12  are considered to be essential. 
     FIG. 2 contains a cut-away side view of fiber  10  of FIG. 1 as it is filled in core  14  with an active material  20 , for example a liquid, that is used to modify the optical parameters of the fiber. Material  20  may be introduced into fiber  10  using any suitable process, such as by using suction pressure or capillary action or by forcing the material into the hollow core using compressed air or an inert gas. Other suitable processes would be used to fill hollow core  14  with a solid material. The particular composition of material  20  is of no concern for the fabrication process of the present invention. Once material  20  has been introduced, fiber  10  is collapsed, in accordance with the present invention, to trap material  20  within fiber  10  and reduce the outer portions of high index cladding ring  12  to form solid core regions. 
     FIG. 3 illustrates fiber  10  after this collapsing operation has been performed. A heat process may be used, applied to end sections  22  and  24  of fiber  10 , to perform this function. In one exemplary process, a heat source, such as a tungsten filament, is moved along end sections  22  and  24  with a variable power so that fiber  10  is tapered down and collapsed on either side. As an example, the heat source is moved over a distance of 7 mm, with a maximum power of 20.5 W. As shown, the collapsing process results in forming a first end coupling section  26  where high index cladding ring  12  has been collapsed to form a first high index core region  28 . In a similar manner, a second end coupling section  30  is formed, where high index cladding ring  12  is collapsed to form a second high index core region  32 . In a preferred embodiment of the present invention, the collapsing process is performed to create an adiabatically tapered section  34  between first coupling section  26  and central region  36  of fiber  10 , and a similar adiabatically tapered section  38  between second coupling section  30  and central region  36 . These adiabatically tapered transition regions  34  and  38  may be partially filled with active material  20  and may also therefore contain a bubble of any gas that was present in the fiber during the collapse process. However, by controlling the collapsing process to create an adiabatic transition, the mode field of the light propagating in first high index core  28  will evolve with low loss as the signal propagates through regions  26  and  34  into central region  36 . Similarly, the presence of an adiabatic taper in region  38  between central region  36  and second high index core  32  will effectively lead to low-loss mode evolution as the optical signal exits active material  20 . 
     FIG. 4 illustrates the collapsed fiber section  10  of FIG. 3 as it is conventionally coupled to separate sections  40  and  42  of transmission fiber. Fusion splicing is an exemplary process, well known in the art, that may be used to join the fiber  10  to fiber sections  40  and  42  after cleaving fiber  10  to provide flat surfaces at either end. In a preferred embodiment of the present invention, the width of high index ring  12  is chosen so that when fiber  10  is collapsed the diameter of high index core regions  28  and  32  will essentially match core regions  44  and  46  of fibers  40  and  42 , respectively. This matching thus provides relatively efficient, low loss coupling of the optical signal into and out of fiber device  10 . Moreover, the resultant symmetry present in collapsed fiber section  10  ensures that low polarization dependence has been introduced in the signal as it propagates from first coupling region  26  through central region  36  and exits through second coupling region  30 . 
     Two specific examples of the application of the hollow core fiber described above will be discussed. In the first example, a hollow core fiber with a highly nonlinear material sealed inside can serve as a compact optical switch where, in addition, a relatively low optical power level can be used for the switching operation. Such highly nonlinear materials include chalcogenides that have a nonlinear coefficient n 2 , which is typically two to three orders of magnitude larger than that of silica. An optical phase shift is induced as a result of self-phase modulation when light propagates through the nonlinear material. This phase shift is defined by Δφ=(2πL/λ)n 2 I, where L is the length of the fiber containing the nonlinear material, I is the optical intensity of the propagating light, and λ is the wavelength. Because n 2  is much larger for chalcogenides than for silica, a π-phase shift can be induced using a combination of a shorter length of fiber and a lower power level, as seen from the above equation. An optical switch can be designed with the hollow core fiber containing the nonlinear material in a number of configurations, including a Mach-Zehnder interferometer and a Sagnac interferometer. 
     A second example involves a hollow core fiber containing an electrically-switchable material such as a liquid crystal. Such a device can be used, for instance, as a modulator. An electric field applied across the fiber core will produce a change in the refractive index of the material. By choosing a material with an index that changes from a value above that of cladding ring  12  to a lower value as the applied electric field changes in magnitude, the distribution of the optical field propagating in the fiber changes from being predominantly in the filled core to cladding ring  12 . If the absorption coefficients of the filling material and cladding ring  12  are different, the optical field will thus be attenuated to different extents, depending on the applied electric field. 
     It is to be understood that although the characteristics and advantages of the present invention have been set forth in the foregoing description, the disclosure is illustrative only, and changes may be made in both the device and method of making the device while remaining within the scope of the claims appended hereto.