Single crystalline structure material having low material absorption and fabrication method therefor

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SINGLE CRYSTALLINE STRUCTURE MATERIAL HAVING LOW MATERIAL ABSORPTION AND FABRICATION METHOD THEREFOR

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 Figure 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 l/λ⁴. 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 siliconoxygen (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 Figure 2A. A comparison of the

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WO 2004/005203 PCT/US2002/032003 modal dispersion of transmitted signals in multimode and single-mode fibers is illustrated in Figure 2B. 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 SiTe₂ 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 SiO2Te_x where x is in the range of 1/3 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

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WO 2004/005203 PCT/US2002/032003 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

Figure 1 is a graph illustrating the transmission loss per wavelength in a standard mode silica optical fiber as the signal propagate in the fiber.

Figure 2A is another graph illustrating the loss mechanism on a spectral wavelength of standard mode silica optical fibers.

Figure 2B is a comparison of the modal dispersion of transmitted signals in multimode and single-mode fibers.

Figure 3 is a flow chart of a process according to the present invention.

Figure 4A is an exploded view of the crucible and the substances as part of the loading process according to the present invention.

Figure 4B is an assembled view of the crucible and the substances for Fig. 4A.

Figures 5A-5C show the process of enclosing the crucible in the envelope according to the present invention.

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Figure 6 is an enlarged view of the resulting aggregate material with a broken crucible.

Figures 7A and 7B are scanning electron microscope photographs showing the aggregate of single crystalline structures at 200 and 20 micron resolution respectively.

Figure 7C is a micro-spectrophotograph of single crystalline structures after processing at 5 micron resolution.

Figure 8 is a graph of the X-Ray Diffraction showing the structures in the powder resulting material.

Figure 9 is a FTIR transmission response of sixteen crystals of the material of

Figures 7C and 8.

Figure 10 is a FTIR transmission response of three of the sixteen crystals of Figures 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process as illustrated in Figure 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 Figures 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 Figures 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 Figure 4B.

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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 Figure 5A. The envelope 50 is neck down at 52 and receives a tube 54 as shown in Figure 5B. 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 Figure 5C. 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 twostep 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 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

CA 02491964 2008-07-18 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 he 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 file characteristics of the oven.

The example was used to form hydroxyl ion resistant silicon dioxide. For example, the molar proportions were SK^Te^. 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 Figure 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 SiOîTe^ distribution of in the aggregate is shown in the Figures 7A and 7B from a scanning electron microscope at 200 and 20 micron resolution respectively. Figure 7C shows a micro-spectrophotograph of single crystalline structures of SiOzTe^ 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 in figure 8, the identified crystal structures are: alpha- low quartz (SiO₂), alpha- low cristobalite (SiO₂), 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.

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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 highpressure 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.

Figure 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.

Figure 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 SiO2Te4/3 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 SiC>2Te_x where x is in the range of 1/3 to 5/3. The single crystalline material may be other silicon based materials for example silicon and telluride. The above process was conducted for SiTe2 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

CA 02491964 2008-07-18 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 also H₂O and H resistant. Thus, the present material may be used as a barrier on substrates, such as metal, ceramic or other surfaces, to protect against OH, H2O 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, Sept. 2000.

2. Infrared Fiber Optics, Naval Research Laboratory, Washington, D.C.

3. Ranka et al., US Patent No. 6,400,866 (June 4,2002).

4. P. Fernândez de Cordoba, et al., La nueva generaciôn de fibras ôpticas, El Pais, June 5,2002.

5. E. G. Doni-Caranicola, et al., Use of single SiTej crystals with a layered structure in optical filter design, J. Opt. Sôc. 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 (SiTeJ, Jpn. J. Appl. Phys., Vol. 34, pp. 3192-3197 (1995).