Abstract:
A method of forming a single crystalline structure having a substantially linear response at least over the wave lengths of 1,200 to 1,700 nanometers, the resulting structure and its use as an optical media or a barrier coating. Thus, maximum obtainable optical transmission with zero attenuation is provided. There is no intrinsic material absorption.

Description:
This application claims the benefit of Provisional Application No. 60/393,829 filed Jul. 8, 2002. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates generally to an optical media or material capable of transmitting an optical signal and, more specifically, to such a material and a method of making a material which is hydroxyl ion (OH) and hydrogen (H) resistant. 
   Silicon dioxide glass or silica is one form of glass used in optical fibers because of its clarity. Other optical materials including silicon have been used. Silicon based fiber has transmission losses. These transmission losses have three components: OH absorption, Rayleigh scattering and the Urbach tail. 
   Silicon based material is a hydrophilic material, which absorbs OH. This absorption produces transmission losses. The transmission losses in general are shown as graph  10  in  FIG. 1  from Ref. 1. That is a graph of transmission losses as a function of wavelength. OH absorption produces the peak at approximately 1400 nanometers, which is approximately one-half of the fundamental OH mode. The Rayleigh scattering effect is illustrated by curve  12 . Rayleigh scattering is proportional to 1/λ 4 . Thus, the Rayleigh scattering is wavelength or λ dependent. The scattering comes from the non-uniformities in the glass, which is disordered by its nature, even though the purity and homogeneity are carefully controlled during manufacturing. Light will scatter from any point where the refractive index varies. The Urbach contribution, as illustrated, produces the Urbach tail  14  beginning at approximately 1,600 nanometers. This results from the vibration of the silicon-oxygen (S—O) bond. The solid line  16  running across the bottom represents the sum of the Rayleigh and Urbach contributions, which may be the clarity limit in the silicon based glass. 
   Spatial spreading of light along the path of propagation is known as dispersion. Appropriate doping can be used to control dispersion. The dopant changes the index of refraction of the fiber by raising the index refraction. Confinement process is similar to internal refraction. Which dopant to use and how it is added is used to optimize all of the parameters associated with high capacity optical transmission systems. The particular configuration will determine the optimization of the interplay between dispersion and non-linearity. 
   The dependence of the loss mechanism on spectral wavelengths of silica standard (single) mode fibers SMF is illustrated in  FIG. 2A. A  comparison of the modal dispersion of transmitted signals in multimode and single-mode fibers is illustrated in FIG.  2 B. See Ref 2. 
   Historically, optical systems have been designed around these limitations of the optical fiber by applying certain modifications and optimizations such as such as dispersion compensators, in-line amplifiers, etc. So, to reduce the transmission loss of fibers, various schemes have been used. These include cladding the basic fiber. Refs. 3 and 4 show two of the latest treatments to reduce intrinsic fiber loss. 
   Ref. 5 and Ref. 6 were one of the earliest attempts in doping silicon to produce a relatively high transmittance optical filter at desired wavelengths with relatively inexpensive cost, but still susceptible to water, as it is the case in the previous two Refs. Ref. 6 is the process used in Ref 5. Si and Te were heated at 1075° C. for 72 hours. The resulting structure of SiTe 2  had to be kept in a vacuum to prevent decomposition in the atmosphere through the interaction with water vapor. These are either costly in material cost, and or as well in the cost of manufacturing. More recent analysis is presented in Ref 7. 
   The present invention is a method of forming a single crystalline structure having a substantially linear response at least over the wave lengths of 1,200 to 1,700 nanometers, the resulting structure and its use as an optical media. Thus, maximum obtainable transmission with zero attenuation is provided. There is no intrinsic material absorption. 
   For silicon base materials, the method produces a hydroxyl ion (OH) resistant silicon material. The transmission versus wavelength response is flat with no absorption peaks between 1,000 nanometers to the Urbach tail at 2,000 nanometers, at a minimum. There is no second harmonic of the hydroxyl ion vibration peak at 1,400 nanometers. The Rayleigh scattering has been substantially eliminated. 
   An example of a silicon based material produced by the present method is a silica-tellurium single crystalline structure. The structure is SiO 2 Te x  where x is in the range of ⅓ to 5/3. The silica and tellurium structure includes twin crystal structures. The twining angle is 90 degrees. The method also includes silicon-tellurium single crystalline structures. 
   One method of the invention includes inserting two substances into a crucible and sealing the crucible in an envelope. The two substances are in an oven at a temperature and time sufficient to create a single crystalline material of the two substances having a substantially linear response at least over the wave lengths of 1,200 to 1,700 nanometers. 
   Another method includes inserting the two substances into a substantially spherical crucible. The crucible is sealed in a substantially spherical envelope. The two substances are heated in an oven at a temperature and time sufficient to create a single crystalline material of the two substances. Heating is carried out for a sufficient amount of time that all of the inserted material is converted to a single crystalline material of the two substances. The opening in the crucible should be large enough to receive the substances while maintaining the crucible spherical. For example, the diameter of the crucible is at least twice the diameter of an opening of the crucible through which the substances are inserted. 
   The resulting material of both methods are an aggregate of single crystalline material. The resulting material of either product may then be processed into an optical media. The material may also be used as a protective coating on metal or ceramics. This may be a crystal, wafer, rod or a fiber. No cladding or other treatment is necessary to obtain the transmission characteristics briefly described. 
   These and other aspects of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph illustrating the transmission loss per wavelength in a standard mode silica optical fiber as the signal propagate in the fiber. 
       FIG. 2A  is another graph illustrating the loss mechanism on a spectral wavelength of standard mode silica optical fibers. 
       FIG. 2B  is a comparison of the modal dispersion of transmitted signals in multimode and single-mode fibers. 
       FIG. 3  is a flow chart of a process according to the present invention. 
       FIG. 4A  is an exploded view of the crucible and the substances as part of the loading process according to the present invention. 
       FIG. 4B  is an assembled view of the crucible and the substances for FIG.  4 A. 
       FIGS. 5A-5C  show the process of enclosing the crucible in the envelope according to the present invention. 
       FIG. 6  is an enlarged view of the resulting aggregate material with a broken crucible. 
       FIGS. 7A and 7B  are scanning electron microscope photographs showing the aggregate of single crystalline structures at 200 and 20 micron resolution respectively. 
       FIG. 7C  is a micro-spectrophotograph of single crystalline structures after processing at 5 micron resolution. 
       FIG. 8  is a graph of the X-Ray Diffraction showing the structures in the powder resulting material. 
       FIG. 9  is a FTIR transmission response of sixteen crystals of the material of  FIGS. 7C and 8 . 
       FIG. 10  is a FTIR transmission response of three of the sixteen crystals of FIG.  9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The process as illustrated in  FIG. 3  includes preparing the mixture of the two components at  20  which are to form the single crystalline material. The prepared material is then inserted in a crucible at  22 . The crucible is sealed in an envelope at  24 . The material is heated at  26  at a temperature and time sufficient to cause a single crystalline material of two substances to form. The time and temperature should be sufficient to cause all the material to form the single crystalline structure. The material is removed from the crucible in an envelope at  28 . The single crystalline material can then be processed into an optical transfer medium at  29 . 
   An example of a crucible is illustrated in  FIGS. 4A and B  as a hollow ball or sphere  30 . The ball or sphere  30  has at least one opening  31 . In the experiments run, the commercially available crucible  30  had a second opening  32 . Since a second opening  32  was in the crucible  30 , both openings  31  and  32  must be closed off in order to maintain the materials in the crucible. For the experiments run, as illustrated in  FIGS. 4A and B , one sheets of material  33  provided to cap opening  32 . For example, the sheet  33  may be gold. If the second opening  32  does not exist, sheets  33  can be eliminated. Sample  34  is then put into crucible  30  and covered by sheet  35  and  36 . For example, the sheets  35  and  36  may be silver or gold. Finally, the total crucible  30  is wrapped in two sheets  37  and  38 , which may be silver. The resulting structure is illustrated in a cross-section in FIG.  4 B. 
   The opening  31  in the crucible  30  should be large enough to receive the substances while maintaining the crucible spherical. 
   It should be noted that the material for the crucible  30  may be quartz, or gold, or silver, or other materials. Also, the covering materials  33 , 35 , 36  may also be quartz gauze, for example. 
   The wrapped crucible  30  is then placed in an envelope  50 , as illustrated in FIG.  5 A. The envelope  50  is neck down at  52  and receives a tube  54  as shown in FIG.  5 B. The interior of the envelope  50  is evacuated. The tube is removed from the envelope  50  and it is sealed at  56  as shown in FIG.  5 C. The resulting structure is a generally spherical shape which resembles a tear drop. The processing of the envelope  50  to form the down  52  and closing it at  56  is performed with heat in a two-step method and sufficiently slow as not to preheat or affect the material in the crucible  30 . The envelope  50  may be quartz, for example. 
   As an example, the crucible  30  may be a ball or sphere having a diameter Dc of approximately 12 millimeter with at least one opening  32 , 33  having a diameter of approximately 3.5 millimeters. The diameter of the crucible Dc should be at least twice the diameter of the opening Do so as to maintain the spherical shape of the crucible. The resulting envelope  50  may have a diameter De of approximately 22 millimeters and a height of 50.8 millimeters. The thickness of the envelope  50  maybe approximately 1 millimeter. 
   Envelope  50  with a crucible  30  and the sample  34  therein is then inserted into an oven. It is heated at a sufficient temperature and time to create a single crystalline material of the sample or two substances. The time should also be sufficient such that all of the material forms a single crystalline material. The structure is an aggregate of the single crystals. 
   To continue the example, the sample  34 , crucible  30  and envelope  50  may be encased in a canister prior to being inserted into the oven. For this experiment, it was placed in a canister of nuclear industrial grade pipe steel. This is to protect the oven from any debris during the heating process. Also, it has been found that the canister extends the cooling time since it is also heated. 
   The canister is inserted into a cold oven. The oven was set, for example, to 800 degrees centigrade. The material was then cooked for five hours and then shut down to cool off. The cooling off period was until it was cool to the touch. This cool period was approximately 10 hours. The envelope  50  was cracked open. The temperature may be in a range of approximately 700 to 1000 degrees centigrade and the time in a range of approximately 3.5 to 7 hours. The temperature range may be above and or below the ionization temperature of the substances and be sufficient to generate single crystalline material. The specific time and temperature may depend on the material of the sample and the characteristics of the oven. 
   The example was used to form hydroxyl ion resistant silicon dioxide. For example, the molar proportions were SiO 2 Te 4/3 . As an example in making one gram of final product, 0.260 grams of silicon dioxide is combined with 0.38 grams of tellurium. The mixture is prepared by putting the two substances in, for example, an Agate mortar. The processed material is then formed into a tablet in a press. All or a portion of the tablet may be inserted into a crucible. For the time and temperature given, half of the tablet was used. 
   The results are illustrated in FIG.  6 . Crucible  30  is shown as dark gray and the aggregate of the resulting material  60  is also shown in the ball as well as outside the ball. The aggregate had a whitish/grayish/brownish coloration with no distinct indication of separate silica and tellurium. The aggregate was somewhat brittle with crystalline surface observed in a microscope inspection. The single crystalline in a structure was twined. It had a twining angle of 90°. 
   Examples of the single crystalline SiO 2 Te 4/3  distribution of in the aggregate is shown in the  FIGS. 7A and 7B  from a scanning electron microscope at 200 and 20 micron resolution respectively.  FIG. 7C  shows a micro-spectrophotograph of single crystalline structures of SiO 2 Te 3/4  at 5 micron resolution after material scraped from the aggregate and then crushed or powdered. The black box is 5 microns on each side for a point of reference. The average micro-crystal size was 1 micrometer. 
   A x-ray diffraction test was performed to determine the structures present in the powder single crystalline material. As is noted, the identified crystal structures are: alpha-low quartz (SiO 2 ), alpha-low cristobalite (SiO 2 ), and an unidentified material structure. The closest recognizable material crystal structure to the unrecognizable material crystal structure based on a rerun of the results of the diffraction test using a greater variance is that of zinc oxide (ZnO). A comparison of the lattices of alpha low quartz, alpha low cristobalite and zinc oxide shows that the zinc oxide is hexagonal pyramidal, twined base to base while quartz is also hexagonal and cristobalite is cubic or tetragonal. 
   It should also be noted that quartz and cristobalite are tektosilicates. In nature they require temperatures above 1400 degrees centigrade and extreme pressure to form. The present method was performed well below this temperature and in a vacuum. 
   The resulting material can then be powdered or processed through a high-pressure press into a thin wafer, rod, cable or fiber form. No high heating step is required. Alternatively, it may be melted into a pre-form of a desired shape, pulled or further processed like other silicon dioxide materials. The single crystalline structure will not be altered by the pressing or the melting. 
     FIG. 9  shows the loss transmission of 16 samples of the above experiment as a function of wave number measure by an FTIR. The vertical scale is not a continuous percentage of loss, but is intended to show the substantially linear and/or flat response. Each vertical scale mark is 5%. Wave numbers 7000 to 5000 correspond to the 1,000 nanometer to 2,000 nanometer wavelengths. There is no spike due to the second harmonic of the hydroxyl ion, and there is no Urbach tail. 
     FIG. 10  illustrates the percentage transmission versus wave number for four selected samples. Again, even though the transmittance percentage varies, all of them are substantially flat. The difference in the transmittance is from the processing of the resulting material prior to the transmittance test. When the sample was pressed to a finer powder and less grainy, the transmission improved. An important aspect is that the response is flat and further techniques in preparing the sample for the measurement is expected to result in substantially 100 percent transmittance. 
   The formation of SiO 2 Te 4/3  is a new material produced by the present process, but the process may be used to produce other single crystal compounds of two substances. The material may be SiO 2 Te x  where x is in the range of ⅓ to 5/3. The single crystalline material may be other silicon based materials for example silicon and telluride. The above process was conducted for SiTe 2  and produced similar results. But these are just examples and the process can be used with other substances. 
   It should also be noted that experiments have been conducted using silicon and silicon dioxide with tellurium in a rectangular crucible and a test tube shaped envelope at the same temperatures and times of the above example, but did not achieve the same results. Very few single crystals of the combined material were formed. The generally spherical shape of the crucible and the envelope produced the increased crystallization of all the material. 
   Not only is the material made from the present process OH resistant, but it is H 2 O and H. Thus, the present material may be used as a barrier on substrates, such as metal, ceramic or other surfaces, to protect against OH, H 2 O and H. The material would be powdered as previously discussed and applied to the surface of the metal or ceramics by known techniques depending on the metal or ceramic. This will prevent oxidation, surface defect and cracking of the surface of the metal and defects or cracking of the ceramic. The same is true for integrated circuit substrates and various metallic layers thereon. 
   Although the present invention has been described and illustrated in detail, it is to be clearly understood that this is done by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims. 
   REFERENCES 
   1. Gordon A. Thomas, et al.,  Physics in the Whirlwind of Optical Communications , Physics Today, pp. 30-36, September 2000. 
   2 . Infrared Fiber Optics , Naval Research Laboratory, Washington, D.C. 
   3. Ranka et al., U.S. Pat. No. 6,400,866 (Jun. 4, 2002). 
   4. P. Fernández de Cordoba, et al.,  La nueva generación de fibras ópticas , El País, Jun. 5, 2002. 
   5. E. G. Doni-Caranicola, et al.,  Use of single SiTe   2    crystals with a layered structure in optical filter design , J. Opt. Soc. Am., pp. 383-386 (1983). 
   6. A. P. Lambros, et al.,  The Optical Properties of Silicon Ditelluride , Phys. Status Solidi (b), Vol. 57, No. 2, pp. 793-799 (1973). 
   7. Kazuhisa Taketoshi, et al.,  Structural Studies on Silicon Ditelluride  ( SiTe   2 ), Jpn. J. Appl. Phys., Vol. 34, pp. 3192-3197 (1995).