OA16913A

OA16913A - Isotopically altered optical fibre.

Info

Publication number: OA16913A
Application number: OA1201400242
Authority: OA
Inventors: James Dalton BELL
Original assignee: James Dalton BELL
Priority date: 2011-12-30
Filing date: 2012-02-01
Publication date: 2016-01-18

Abstract

An optical waveguide having a cladding layer formed of high-purity glass, or a cladding layer formed of high-purity isotope-proportion modified glass, and with a core of high-purity isotopeproportion-modified glass with the index of refraction of the core glass greater than the index of refraction of the cladding glass, said high-purity isotope-proportion-modified core material having a Si-29-isotope proportion at most 4.447 % Si-29 (atom/atom) of all silicon atoms in said core, or at least 4.90% of Si-29 (atom/atom) atoms in said core, or having a Ge-73 isotope proportion of at most 7.2% Ge-73 (atom/atom) of all germanium atoms in said core, or at least 8.18% of Ge-73 (atom/atom) of Germanium atoms in said core region.

Description

Isotoplcallv Altered Optical Fiber

Technlcal Field

This invention generally relates to an Isotoplcally altered optical fiber, and Is specifïcally concemed with a silica fiber either depleted of or enriched with Si-29 isotope atoms, or depleted of or enriched with Ge73 isotope atoms.

Background Art

Optical loss Is a limiting factor ln the design and construction of optical networks and links, which typically Inciude hundreds of kliometers of sillca-based optical fiber. Optical losses ln silica fibers are predominantly caused by two factors: (1 ) Rayleigh scattering, which falls off as a fonction of 1 /A⁴ (where λ is wavelength) and which dominâtes for shorter wavelengths; and (2) infrared absorption by the silica, which dominâtes for longer wavelengths. Typical germania (GeO₂) doped silica-core fibers hâve losses of 0.189 db/km to 0.200 db/km. between 1510 nm and 1610 nm.

There was a previous attempt to develop an optical fiberwith lower transmlsslvlty losses by means of Isotoplcallyaltering the fiber régions. See U.S. Patents 6,810,197 and 6,870,999. The improvement ln loss was limited to about 0.145 to 0.155 db/km, and was accomplished primarily by shifting the wavelength of minimum optical loss to about 1670 nm, and partly by changing the index-of-refraction dopant from germania to Oxygen-17, although the inventors may not hâve recognized the reason for the réduction ln loss due to their use of Oxygen-17.

Fiber-optic sclentlsts and engineers hâve not recognized that Si-29 isotope Is the source of nearty ail of the variation of Index of refraction of silica from 1.0000, with natural-lsotope-proportlon Oxygen-17 providing a minor Increase at normal (naturel) levels. Llkewlse, such sclentlsts and engineers hâve not recognized that the Ge-73 isotope (the dopant which, Is normally used) increases the index of refraction of fused silica ln Its naturel Isotoplc proportions, from 1.46 to ebout 1.47. It Is also not recognized that It is the SI-29 dopant which is responsible for the large majority of Rayleigh scattering présent in existingtechnology optical waveguides. They do not the Si-29 as a dopant since is a naturellyoccurring stable isotope of sllicon.

Thus, a réduction ln SI-29 isotoplc proportion ln silica of, say, a factor of 100 (from nature's 4.67% atom/atom to 0.0467% atom/atom) will resuit ln a material with an Index of réfraction of 1.005, and a réduction by a factor of 33 will resuit in a material with an index of réfraction of 1.015. These two materials, with a différence ln Index of refraction of 1.015-1.005 » 0.010, are well within the right range to become the cladding, and core, respectively, of a new fiber.

Slmilarly, U.S. Patent 6,490,399 describes the substitution of silicon-30 for silicon-28 isotope, which has a similar effect of moving to the right of the graph the intrinsic IR absorption line. This results in opening up a new région of useable transmission. See Figure 2 showing a région labeled B from about 1610 through about 1710 nanometers wavelength, for the substitution of both Si-30 for Si-28, and O-18 for O16.

U.S. Patent 6,810,197, in its Summary of the Invention (column 1, line 58 to column 3, line 14) describes a réduction In the number of needed amplification stations for a cross-Atlantic link, by 11 unlts, due to an Increase in the possible Inter-ampllfier spaclng from 125 kilometers to 156 kllometers. This benefit Is almost certain to be illusory in practice, however. Any practical link that transmits from 1610 through 1710 will also be deslgned to employ the 1510-1610 band, and the isotopic substitution will not appreciably asslst the fiberis transmission in most of the 1510-1610 nm band. Since the same amplifier station that amplifies the 1610-1710 band wiil also be the one to amplify the 1510-1610 band, proper operation on the latter band will require maintenance of the same 125 kilometer Inter-amplifier spaclng as Is currently required. Thus, the only useable improvement will be a broadening of the useable bandwidth on which signais may be sent. In other words, substitution of Sl-30 for Si-28 and ΟΙ 8 for 0-16 does not actually enable any savings due to réduction In waveguide loss, and even the widened band (including the 1610-1710 région) is likely to be bénéficiai only In links in which wavelength division multiplex (WDM) signais aireadyoccupyail ofthe 1510-1610 bandwidth.

Similariy, the reference In Patent 6,490,399 to Patent Abstracts of Japan, JP-A-60090845, describes a method of employing a deuterium rinse of the porous SiO₂ preform to replace existing -OH groups with OD groups, thus shlfting their absorption bands (including 1400 nm) to much longer wavelengths longerthan 1710 nm. Yet, that technology Is described as costly, In part because fi ber manufacturera hâve aiready done a good job of reduclng -OH content, in part by continuai Improvement of the Cl₂treatment which was described In Patent 3,933,454, columns 7, line 1 to 8, line 68.

However, this merely means that the -OH absorption spectrum, especially at 1400 nm, see Flg.2, Is sufficientiy low compared to the Rayleigh Scattering minimum line, see Fig. 3, so as to make further Improvement seemingly without benefit. The instant invention, by means (In part) of reduclng Sl-29 concentrations by large amounts, up to a factor of 50-100 or more, has the effect of greatly reduclng the altitude of the Rayleigh Scattering minimum by a large, related amount, which will enable extra utilîty for a deuterium (D₂) rinse as was described in JP-A-60090845.

Therefore, an embodiment of the instant Si-29-reducing invention will likely hâve further unanticipated benefits from both a deuterium (D₂) rinse, as well as a substitution of Si-30 for Si-28, or a substitution of O-18 for O-16, or both. A full Implémentation of these modifications may resuit In an optical waveguide that has a transmission bandwidth from at least 1230 nm to about 2000 nm at a loss of about 0.01 db/km or lower, and so can achieve trans-Atlantic transmission with no or, at most, one amplifier station. This attempt shifted the line representing IR Absorption (see Figs. 4 and 5) to the right on these graphs. This had the effect of reducing the minimum absorption, caused by the sum of IR absorption, Rayleigh [scattering], and UV to be slightly lowered, with the resulting transmission bandwidth somewhat broadened.

The Instant invention, however, attacks not only the IR Absorption line, but in fact also the Rayleigh Scattering line. See Figure 10, long-hatched line, labeled Rayleigh scattering.

A réduction In the quantity of Si-29 scattering centers by a factor of X will reduce the altitude of the Rayleigh scattering line by a factor within the range of X and the square-root of X. This will resuit, for a factor of 33 réduction, In Si-29 concentration, In between a factor of 33 and the square root of 33 (about 5.9) réduction In atténuation due to Rayleigh scattering effects. This dramatically reduces the overall atténuation seen In the 1310 nm band, as well as In wavelengths up to approximately 1650 nanometer.

Of course, a combination of a portion of the features of patent 6,810,197, the substitution of O-18 for O-16 in both the core and réglons of the cladding near the core, and the dramatic réduction in SI-29 isotope proportion down to and through a factor of 100 réduction (to 0.0467% of Si-29 atom/atom) combines to cause a remarkable réduction In overali atténuation due to the signal's passage through the optical waveguide.

The authors of patent 6,810,197 were under the Impression that at least some 0-17 Isotope was necessary to provide useful benefits, co-su bstituting with Oxygen-18 forthe majorityof Oxygen-16that would ordinarily be présent In non-isotopically-modified fiber. See, for examples, Claims 1,3,4, and 9 In patent 6,810,197.

In contrast, this instant Invention spécifiés the presence or absence of a 0-17 isotope, but In proportions sufficiently lower than to overlap the claims of patent 6,810,197 or other patents or applications.

Persons familier with the art of fiber optic waveguide design, i.e. fiber optlc scientists and engineers, will be able to define, for a given transmission wavelength and core diameter, the necessary index of refraction différence forltto function as a single-mode transmission medium, or alternatively as a muitimode transmission medium.

The fused silica index of réfraction may be adjusted from the natural-lsotope-distribution value of about 1.46, to vlrtually any value down to 1.0000, depending on the réduction in the proportion of Si-29 isotope achieved. So, the example above of the indices of refraction being 1.015 and 1.005 is exemplary and without limitation.

Patent 6,128,928 describes the anti-free-oxygen benefits of a small doping of germanium oxide added to the cladding or inner-ciadding région of an optlcal fiber. In that context, however, the index-ofrefraction-raising effect of the Ge-73 isotope (which the author of Patent 6,128,928 does not recognize as the very large majority of the source of the index-of-refraction-ralsing-effect) Is a drawback. The inventor of the instant invention, Instead, spécifiés the addition of ONLY (or a large majority) of Germanium atoms other than Ge-73 isotope atoms, in order to obtain the same benefit of patent 6,128,928 without raising the index of réfraction. The author of the 6,128,928 patent cleariy did not anticipate the possibliity that an isotopicaily-modified sample of germanium oxide could be used, instead of a natural-lsotope sample.

The Inventor ofthe instant invention was familiarwith the principles of optics and ofwaveguldes, having taken a Physics course numbered ’8.03' at MIT in the fall of 1977. (

In early 2007, the Inventor had the opportunity to read a 1979 book on the very highly technical aspects ofoptical-fiber construction and use. In November/December2008, the inventor had the opportunityto read Coming Glass Works v. Sumitomo Electric U.S.A., both the district court case at 671 F. Supp 1369 (S.D.N.Y. 1987) and the Fédérai Circuit appeal case at 868 F.2d 1251 (Fed.Cir. 1989). This provided a very extensive discussion of the history of opticai-fiber research and details as to its construction and design.

The Inventor also happened to obtain a list of naturaily-occurring Isotopes of each element, about 256 In total. Sillcon consists of about 92% SI-28,4.67% SI-29, and 3.1% Si-30. Germanium is about 7.8% Ge-73. A given nucleide (an isotope’s nucléus) possesses a property called 'spin¹ (actually, 'electromagnetic spin’) if it had either an odd number of protons or an odd number of neutrons. Thus, of silicon's Isotopes, only the Si-29 (4.67% atom/atom) had ’splrï, and only the Ge-73 (7.8% atom/atom) had 'spin*.

’Splrï may be thought of as a permanent wobble caused by the fact that there remains a single, unpaired nucléon présent. It causes a vibration ofthe (positivety charged) nucléus, making that nucléus behave something like a tiny bar-magnet. This spin could Is used in Nuciear Magnetic Résonance analysis, most commonly with Hydrogen-1 atoms, and In Magnetic Résonance Imaging. Isotopes are also used, occasionally, as ’tracers*, to follow the mechanism of chemical reactions.

By reading the Coming case, the Inventor knew that the addition of about 8% (weight/weight) of Germania (Ge02) to silica (Si02) had the effect of raising the index of refraction of pure silica (at 1.4584) to about 1.466. But why, the Inventer wondered, did It do so? It occuned to me that since the silicon atoms were only 4.67% spin-containing, and the replacement germanium atoms were 7.8% Ge-73, the Inventer thought that maybe the presence of electromagnetic spin-contalning atoms was the underlylng reason that materials even hâve an index of refraction greater than that of air or a vacuum (1.000), and It tums out that the Inventor was right. Even then, the inventer understood that the Inventer did not know if a given Ge-73 atom had a greater effect, overall, on index of refraction than a SI-29 atom, but that was a question that the Inventor could not then answer.

But, the realizatlon that Si-29 may be the underlylng reason that silica has an Index of refraction over 1.000 ied to a number of Ideas In quick succession:

1. You could add SI-29 to silica, rather than adding Ge02 to silica, to increase its Index of refraction over that of the cladding région.

2. You could decrease the proportion of Si-29 Isotope atoms In the cladding région, rather than Increasing them in the core, and2thus produce the dlfferential in index of refraction necessary to2have a functlonlng optical waveguide.

Either of these Ideas is interesting, but they would only hâve provided a small Increase in benefit forthe opticai-fiber manufacturlng industry. Either Idea would slightly reduce the optical loss over germaniadoped opticai fibers, but In both cases the veioclty factor would romain close to the 68% of c characteristic of exlsting optical fibers.

The blg question was: how low could the index of refraction of the core and cladding be brought down and the core and ciadding still functlon as a waveguide? As far as Is known, the only limit was that It was impossible to lower the Index of refraction of the cladding material to a value of 1.0000, the same value as a vacuum. And, with such a cladding, the core would probably hâve to hâve an index of réfraction about 0.008 larger, and thus it would be 1.0080.

The resulting fiber would hâve a velocity factor of 1/1.008, or 99.2% of c. The Inventor realized that would be very vaiuable for users of fiber optlcs to be able to accelerate the signais from the existing veiocity factor of 0.68 to near 0.99. The Inventor was notawareofthe invention of an opticai fiber with a velocity factor of 98-99% of c.

But this was not surprising since there Is llttle need for Isotopes of various éléments and thus science and Industry only rarely attempt to separate isotopes of the same element.

In the field of Chemistry, tracer (stable) Isotope-tagged chemicals are sometimes used to analyze chemical reactions

Three patents were granted in the early-2000 time frame, one to Deutsche Telekom and two to Coming, on the very subject of s table-isotope enhancement. But the only isotopic ratios they were taIkîng about modifying were Si-28 versus Si-30, or 0-16 versus 0-18, and to a smallerextent 0-16 versus 0-17. Si-29 simply was not considered.

The mechanism to generate the necessary Isotopically-modified precursor (SiC14; silicon tetrachloride) already exists. See the Silicon Kilogram Project (Google Silicon Kilogram Sphere). They separated the sliicon-containing precursor (which was probably either silane (SiH₄) or silicon tetrafluoride (SiF₄)), in Russian gas centrifuges, and converted it Into single-crystal silicon later. Instead, the instant Invention requlres the silane or silicon tetrafluoride tumed Into SiC14, which can be d irectiy used as input to the optlcal-fiber manufacturing process of the same type that Coming patented In 1976.

Development of an optically transmissive material and a waveguide which has much reduced index of refraction represents a great Improvement In the optical arts and satisfies a long felt need of the optical engineers.

Disclosure of Invention

This invention is an Isotope-modified version of Silica (SiO₂) that has an index of réfraction of anywhere below nature’s value of 1.4584. This means that natural-lsotope-proportion silica transmits light at a speed of (1/1.4584) that of c, where c Is defined as the speed of light In a vacuum. (A vacuum has an Index of refraction of 1.0000 exactly, by définition.) The speed of light in a vacuum Is about 299,700 kllometers/second. Light travels through air at about 0.999 c. The graph shown in Fig. 1 is from the 1989 issue of Encyclopédie Britannica, Macropedia, Volume 23, pages 665-666. The thinnest, darkest région shown represents optical glasses that were available in 1880. You can see that no known glasses were available in 1880 with an index of refraction below about 1.45. By 1934, the technology had advanced to a point (see un-shaded région labeled ordinary optical) which included glasses with as low an index of refraction as 1.40.

The rest of the graph (llght-tlnted) shows various kindsof glasses developed slnce 1934. Of these,the fluophosphate and fluoride giasses include réglons with an index of refraction of about 1.32. The reason that currently available glasses do not hâve an index of refraction much below 1,4584 is that, by and large, they are made with natural-lsotope-proportion silica. In otherwords the amount ofSi-29 they contain is nature’s 4.67% of ail silicon atoms, (atom/atom). The inventor has discovered that it is almost entirely the Si-29 (and, to a much smaller proportion, the 0-17) which is responsibie for the fact that the index of réfraction of Silica is greater than 1.0000. Scientists and engineers do not realize this, because they virtually never see isotope-proportion-modified materials. The silica they see always has a Si-29 proportion of 4.67%, atom/atom. Silica with an index of refraction of 1.02 can be made if it has an

Isotope-proportion of 0.20% Si-29, atom/ atom. Silica with an index of 1.01 can be made if it has an Isotope proportion of 0.10% Si-29, atom/atom, etc. The effect Is not precisely linear: these values were selected for purposes of illustration.

More particularly, the instant invention relates to an optical waveguide comprising silica which is présent In an isotopic proportion varied in, and usually heavily depleted of, the sllicon isotope Si-29. A dramatic réduction of the isotopic proportion of Si-29, from the 4.67% (atom/atom) usually found In nature, by a factor of about 50x (to about an isotopic proportion of 0.093% SI-29) wiil resuit In a fused silica with an index of réfraction of approximately 1.010. This contrasts with the index of refraction of 1.46 présent In natural-lsotope-proportion fused silica. It is the object of this invention to provide an optical waveguide with spécifie improvements in waveguide performance, including:

1. Signal velocity factor settable to a value far above the 0,67 value normally associated with silica with an index of refraction of 1.46 (1/1.46 = 0.67). This velocity factor should be adjustable up to at least 0.995, meaning that the optical signais will traverse the core région at a velocity of 99.5% c. Light travels through pure waterat about 0,750 c. Light travelsthoughordinarytypes of glass at about 66 % c. Thus, data signais will be able to travel about 1.5 times faster than in ordînary, conventlonal-technology optical fibers.

2. A very large réduction in opticai loss from the 0.191-0.200 db/km usually seen In germanladoped silica fibers, and also from the 0.160 db/km typically seen In un-doped core silica fibers. This réduction will be at ieast a factor of 5, and perhaps well over a factor of 50 with all isotopemodifications having been added. If the latter value is achieved, and thus a ioss value of 0.0032 db/km is achieved, then fiber signais could be transmitted across the Atlantic Océan with two, one, or even NO Intermediate re-amplification. This réduction of Rayleigh scattering would cover most of the 500nm-1650nm band.

3. Large réductions in chromatic dispersion and pulse-broadenlng, commensurate with the réduction in optical loss, to include most of the visible wavelengths of 400-700 nm, as well as most of the infrared wavelengths of 700-1650 nm.

In one embodiment, a réduction of index of refraction of the claddingto about 1.005 would be combined with an index of refraction of the core at 1.015, resulting In a velocity factor of about 0.985, meaning that the data signais would travel at a velocity of about 98.5 % light travelling in a vacuum. The différence In index of refraction between the core and cladding in this spécifie embodiment is achieved by at least four mechanisms:

1. Réduction in the isotopic proportion of SI-29 In the core région silica to about 4.67% /30, while changlng the Isotopic proportion of SI-29 In the cladding région to about 4.67%/100.

2. Réduction in the Isotopic proportion of SI-29 In both the core and cladding régions to about 4.67%/100, while adding some Ge-73 Isotope (or a larger proportion of natural-isotope germania) to increase the Index of refraction to 1.015.

3. Réduction In the isotopic proportion of SI-29 In both the core and cladding réglons to about 4,67%/100, while adding a proportion of Oxygen-17 isotope atoms, above the proportion of 0.038% found In nature, sufficient to ralse the index of refraction of the core to a level sufficient to malntaln proper waveguîde action.

4. Réduction in the Isotopic proportion of SI-29 In both the core and cladding réglons to about 4.67%, while adding to the core région amounts of both Oxygen-17 Isotope and Germanlum-73 isotope (or both Oxygen-17 Isotope atoms and some germania containing a significant isotopic proportion of Germanium-73) sufficient to ralse the index of refraction of the core to a level sufficient to maintaln proper waveguîde action.

In another embodiment, the index of refraction set points for the core and cladding régions are set to different values, but malntain the relationship so that the différence functions to perform the actions of a waveguîde. They may be set, for example, to a core Index of refraction of 1.04 and a cladding Index of refraction of 1.03.

This cholce reduces the requirement for purifying an isotopic sample of precursor material. The core région need only be made of sllicon atoms reduced in SI-29 content to a factor of 12 times lower than the 4.67% atom/atom found in naturel samples, whereas the cladding région need only be made of silicon atoms reduced In SI-29 content to a factor of 17 lower than 4.67% atom/atom of SI-29 Isotope atoms.

In yet another embodiment, both the core and cladding réglons are constructed almost entirely of germania (GeO₂) which has been isotopically-modified to reduce the normal proportion of Ge-73 Isotope atoms by a factor of 100 and 300, respectively (7.8%/100 and 7.8%/300 atom/atom, respectively, analogous to item 1, above). Slmilariy, In another embodiment, both the core and cladding régions are constructed almost entirely of germania which has been îsotoplcally modified to reduce the normal proportion of Ge-73 to a factor of 300 lower (7.8%/300) but where the Index of refraction of the core is raised by the addition of either a small doping of SI-29 atoms or of 0-17 isotope atoms, or both.

In another embodiment, and to reduce the amount of isotopically-modified material which must be empioyed, the core région Is constructed so as to be surrounded, immediately, by an inner cladding région, and then an outer cladding région, with the possibility that the material empioyed in the outer cladding région will be depleted to a lesser degree in S9-29 isotopes. For example, the core may be set to an index of refraction of 1.015, the inner cladding may be set to an index of refraction of 1.005, and the outer cladding may be made with an isotope proportion of Si-29 (or, altematively, Ge-73) identical to or doser than to the 4,67% atom/atom (for Si-29) or to the 7.8% atom/atom (for Ge-73) proportions of these species found in natural-lsotope proportions found in terrestrial samples. This construction method will tend to minimize the cost ofthe material empioyed. Note, however, that in this technique the diameter ofthe inner-cladding/outer-cladding interface must be sufficiently larger than the diameter of the core/ inner cladding interface, to ensure that no more than an acceptable proportion ofthe light may ’leak* into the outer-cladding région and be iost, thereby increasing the overaii atténuation ofthe fiber. A person skilled In the art of fiber optic engineering and science will be able to mathematically predict, and separately confirm by experiment, that the outside diameter of the inner cladding région is sufficiently larger than the diameter of the core région to lower losses to acceptable levels.

SOURCE OF ISOTOPICALLY-MODIFIED MATERIALS

Silica is derived from silicon-containing precursors depleted in Si-29 isotope atoms. These siiiconcontaining precursors can be of at least three types:

) Enhanced in isotopic proportion of Si-28 isotope atoms to approximately 99.5% atom/atom, and depleted in Si-30 isotope atoms to a very small or negligible amount: Such an isotopic distribution can be expected from the output of a gas centrifuge, selecting the light molécules, SiF< or SiH».

2) Enhanced in isotopic proportion of Si-30 isotope atoms about to 90% atom/atom or more, and depleted in Si-28 isotope atoms to 10% or less. This would also be a gas-centrifuge output product, selecting the heavy molécules ...

3) Depleted in Isotopic proportion of SI-29 isotope atoms from the natural proportion of 4.67% atom/atom, but otherwise not greatly varylng the relative proportion of SI-28 and SI-30 Isotope atoms. In each of these examples the Si-29 proportion is set to a value calculated to achieve a spécifie index of réfraction for the as-deposited silica materiai.

For a typical preferred embodiment, employlng silica with an isotopic distribution of silicon atoms with about 0.1% Si-29 isotope atoms, the main différence between the 90%+ Si-30 isotope constituency, Item 2 above, and the 99.5% Si-28 isotope constituency, will resuit in a broad pass band for IR signal transmissions atwavelengths in excess of 1600 nm. The spécifie extent ofthe broadened transmission spectrum and the loss figures cannot be easily predicted prlor to constructing the fibers, but It will be understood that these extra useable frequencies, specifically at losses well under 0.1db/km, hâve not been anticipated by prior optical waveguide art. It wlil also be understood that these extra useable frequencies hâve not been provided, or foreseen, accompanled by transmissions bands from about 1400 nm which are equally useable due to hyper-low-loss transmissions.

An appréciation of the other aims and objectives of the présent invention and a more complété and comprehensive understanding of it may be achieved by referring to the accompanying drawings and studying the following description of the best mode of carrying out the invention.

Brief Description of Drawings

Figure 11s a graph of refractive Index, n, versus constringency, v, for the range of optical glasses. From Glass in the Modem World by F.J. Terence Maloney, 1968, Doubleday & Company, Inc.

Figure 2 Is a reproduction of Publication 2003/0002834 Fig.1

Figure 3 is a reproduction of Patent 6,490,399, Fig. 1.

Figure 4 Is a reproduction of Patent 6,810,197, Fig 4.

Figure 5 Is a reproduction of Patent 6,810,197, Fig 5.

Figure 6 is a reproduction of Patent 6,810,197 Fig. 9

Figure 7 is a reproduction of Patent 6,810,197 Fig 12.

Figure 8 is a reproduction of Patent 6,810,197 Fig 10

Figure 9 is a reproduction of Patent 6,810,197 Fig 11

Figure 10 is a reproduction of Patent 6,810,197, Figure 2.

Best Mode for Carrying Out Invention

BENEF1TS OF THE INVENTION

Due to the avaiiabiiity of glass materials with an Index of refraction of as little as 1.001 by reducing the SI-29 content from nature’s 4.67% atom/atom to about 0.01%, or even lower, the index of refraction of the core in a waveguide can be approxlmately 1.001-1.002, and thus the Velocity factor* of the signais transmitted through that fiber could be at least as high as (1/1.001 ), or 99.9% c.

Existing optical fiber cables typically employ waveguides that operate with a velocity factor of about (1/1.47), or 68% of'c'. A signal can be transmitted a distance of about 6,000 kilometers, approxlmately the cable length between New York and London, within a one-way delay of about 36 milliseconds. Even employing a currently-proposed route designed to be straighter and more direct, a cable distance of

5000 and a one-way delay of 30 milliseconde is the minimum that current technology can provide.

However, convertlng the fiber to one which has a velocity factor 99.5%, rather than the heretofore typical value of 68%, results ln a one-way delay on the longer route of about 2/3 of 36 milliseconds or 24 milliseconds. Use of that faster fiber on the shorter route, which is forecast to hâve a one-way delay of 30 miiliseconds with ordinary cable, will produce a one-way delay of 2/3 times 30 milliseconds, or 20 milliseconds.

This klnd of différence in signal delay is extremely important for real-time interactive video, server response, database lookup, Internet gamlng, and low-iatency téléphoné communication generally. It also allows database and other servers to be located further away from users for the same delays.

The financial market, such as stock exchanges, will be especially affected. An extra delay of a few milliseconds, each way, could cost a high-volume stock trading firm tens of millions of dollars per month. A réduction by a factor of 2/3 of the delay between New York and London, or between New York and Los Angeles, from 36 miiliseconds to 24 or even 20 milliseconds will more closely tîe not merely a single country, but ultimately the entire world

Internet gamîng Is another use. Today, two usera located ln, say, New York and Sydney, Australie, can be locked ln (bloodless) digital combat, with their computers only leamlng 100 milliseconds laterwhat the opponent did. These delays, while seemingly small, are quite perceptible and substantially affect the flow of the games. A réduction of a factor of 2/3, from 100 milliseconds to 67 milliseconds, In the oneway link delay will provide the minimum theoretically-posslble delay.

OPTICAL LOSS BENEFITS

It has long been accepted thatthe minimum optical loss fora pure-siiica core waveguideatthelossmînimum of about 1560 nm is 0.151 db/km. This was changed somewhat with the possibility of substitution of SI-30 for Si-28, and substitution of 0-18 for 0-16. But, Improvements to below 0.10 db/km still seem hard to achieve, and are considered expensive.

The Instant Invention, by reducing SÎ-29 by factors of 50,100, or more, reduces optical loss by a factor of 10, and quite posslbly a factor of 100 or more. This allows repeater less spans of 5000 kllometer or more to be achieved.

OPTICAL BANDWIDTH INCREASE BENEFITS

The current optical bandwidth, between 1510 and 1610 nanometera, Increases, with the Instant invention, to at least 1450 nm through 1800 nm, and quite possibly 1230 nm to 2000. While full utilization of some of these waveiengths wiil await production of suitable transmitter lasers and detectors, a large fraction of this new région of transmission should be available almost immedîately. In any case, fols new-capacity fiber can be Installed immedîately, operated at the traditional 1510-1610 band, and the wavebands expanded as the receivers and transmitters become available.

However, there Is lîttle value in merely being able to increase the transmission bandwidth of an optical fiber from 1510-1610 nm to include, say, the 1610-1710 nm band, as Patent 6,810,197 does, without the matching fiber amplifiera which would (if they exlsted) amplify that extra bandwidth. Fi ber amplifiera based on erbium amplify approximately in the région of 1520-1565 nm bandwidth, making this the main transmission région. While it would be possible to detect and re-transmlt such signais at each amplifier, this would be expensive and would render nugatory the extra bandwidth. It mlght be cheaper simply to instaii extra fi bers to achieve a larger capacity.

If, Instead, this bandwidth could not only be made available, but also available with a loss of approximately 0.01-0.02 db/km; a far-wider band could be employed, and that even without the use of any fiber amplifiera at ail (or, at most, one détection and re-transmlssion amplifier at approximately the mid-point of the Atlantic.)

The bandwidth that can be used by empioying the instant invention runs from about 1430-1750, about 320 nm, seven times wider than the 45 nm of the 1520-1565 nm band, and that would be for using an ordinary Isotope distribution of SI-28 vs. Si-30, and 0-16 vs. 0-18. By empioying nearly ail Si-30, as well as nearly ali 0-18, the bandwidth made available increases to 1430-2000 nm, or 570 nm, about 13 times wider than the 1520-1565 nm band.

DRAMATICALLY DECREASED MATERIAL-SOURCED DISPERSION FACTOR

A large réduction in the proportion of Si-29 isotope atoms In the waveguide material will resuit in a cornmensurate large réduction of the overall dispersion of the as-manufactured optical waveguide. See Figs. 6 and 7. The portion of the dispersion attributable to the waveguide itself (see Fig. 8) will likely remain the same, and the profile dispersion can be selected to neutrallze, any residual overall dispersion that may remain after other Isotope modification processes are complété. A person familiar with the art of optical fiber waveguide design will be able to define various substitutions of core versus cladding that will accomplish the desired dispersion level.

This Invention contemplâtes a re-designed optical waveguide, with a large réduction In the Isotopic proportion of SI-29 In the core and cladding réglons (approximately between 3x and 10Ox réduction of Si-29 proportion from naturel 4.67% atom/atom of ail silicon atoms présent), which wili resuit in a large change from the curve in the value of Material Dispersion] (core), analogized from Fig. 6, and a much smaller change (If any) In the curve in Waveguide Dispersion], analogized from Fig. 8, Profile D, combined with a 0-8 core/0-16 clad, analogized from Fig. 9.

In other words, the monotonicaliy-rising and straight line of line 4 on Fig. 9 will compensate, In part, for the monotonically-falling and straight line(s) of lines 1-4 in Fig.8. The residual of the addition of these values may be combined with the SI-29-reduced curve, analogous to Fig. 6, line 4, which can be expected to be of far-lower slope than that in Patent 6,810,197.

Judicîously selected, the overall dispersion, analogous to the Total Dispersion] of Fig. 7, will be the linear sum of the Material Dispersion], the Waveguide Dispersion], and the Profile Dispersion], and may be lower than a value of 1.0 ps/nm/km, which is a very large réduction from the value of about 20 ps/nm/km at a wavelength of 1.6 microns on ordinary-isotope '0-16 core/O-16 clad which is shown on Fig. 7.

Flber-optic waveguide scientists and engineers hâve long been familiar with the need for and use of dispersion-compensating fibers (DCF). The purposes of these fibers, which are used in relatively short lengths (as compared to the hundreds orthousands of küometers présent in typlcal optical-wavegulde links) is to oppose, and thus counteract and fully or partially neutralize, a change in the time of arrivai of differing-wavelength portions of a given optical signal. Their use may be criticaI ln order to be able to achieve the maximum bit-rate (technically, symbol rate) that a given actual data-link can carry. The reason is that dispersion tends to blur, in time, the arrivai time of transitions of optical signais which make up a given bit.

For example, a symbol-rate of 40 Gblts/sec (40 billion bits per second) implies a bit-time of (1/40,000,000,000) = 25 picoseconde, if one bit is présent in each optical-signal symbol. This rate, 40 Gblts/sec, Is the fastest commercially-available data-rate employed ln the most recently-installed (or upgraded) data links. But, It does little good to hâve an optical detector capable of detecting a 25picosecond minimum signal time, If that bit (symbol) has been spread in time to, say, 50 picoseconde or more. Since the optical wavelength bandwidth of a 40 Gbit/sec signal will be approximately a minimum of 0.3 nanometers, according to Fourier's Law, an uncompensated conventional fiber with a dispersion of 20 ps/nm/km would add a dispersion-caused blur In time of 25 picoseconde after a fiber length of about four kilometers. It can be seen, therefore, that compensating for this dispersion Is essential to allowing the maintenance of 40 Gbits/ sec even for short distances of fiber.

While a large fraction of optical-fiber dispersion is currently compensated with dispersion-compensating products, the completeness of any such compensation tends to be Iimited to less than 100%. Thus, the better the main fiber Is (the lower its baseline dispersion), the better the ultimate as-compensated optical link’s dispersion can be expected to be. The instant inventlon's prospect of reducing native, uncompensated fiber dispersion by a factor of 10 times or more provides the prospect of a similar as16913 compensated improvement in signai dispersion.

INSTANT INVENTION MAY EMPLOY TECHNIQUES FROM US PATENT #4,435,040

It is expected that optical waveguides built with this invention will also be able to employ the double-clad process described ln U.S. Patent #4,435,040, with certain cautions and modifications. First, It should be remembered that ln existing technology waveguides, the cote and cladding hâve indices of refraction very close to that of naturai-isotope distribution silica, 1.46. The physical wavelength of the optical signal In such a waveguideis, for example, 1510 nanometers divided by 1.46, orapproximately 1050 nanometer. in contrast, in a waveguide whose indices of refraction are about 1.01, the physical wavelength in that waveguide Is 1510 nanometers/1.01, or about 1495 nanometers. Since the behavior of the wave within the waveguide is based on the size of the guide compared to that of the optical wave, it is to be expected that dimensions of waveguide components such as the core and cladding will be sized to a factor of about 1.46/1.01, or 1.45, greater than existing-technology fibers, ail else being equal. Where, for example, patent 4,435,040 spécifiés an effective core diameterof greaterthan 2x4 microns, or 6 microns, this would translate into a core diameter of 8 microns x 1:45, or about 11.6 microns. Simllariy, where patent 4,435,040 spécifiés about a factor of between 0.5 and 0.8 différence between the radius of the core and the radius of the inner cladding, such factors will operate on 11.6 microns, leading to an OD of the inner cladding of about 23.2 microns and 14.5 microns.

Patent 4,435,040 also forecasts that the thickness of the outer-cladding région should be between a factor of about 6 and 8 greater than the core radius, or (6-8) x (5.8 microns), or between 34.8 and 46.4 microns, which would make the OD of the outer cladding région 11.6 + 2(34.8 to 46.6) or between 81.2 to 104.8 microns. Outside the outer cladding région Is allowed to be yet another cladding région, posslbly with an index of refraction equal to that of ordinary silica, or 1.46.

Another caution is that many patents like 4,435,040 refer to changes in the percentage of the index of refraction of a material, for example 0.1% to 0.6% of a value which can be expected to be close to that of naturai-isotope silica, or 1.46. This translates into a change of 0.1% times 1.46, or a différence of 0.00146, to 0.6% of 1.46, or about 0.00876.

But when a silica material Is isotope-modified to greatly reduce SI-29 content, for example to about 1/1 OOth of nature’s 4.67% atom/atom of SI-29, and thus an index of refraction of approximately 1.005, it is not possible to reduce that value (1.005) by 0.00876: The Index of refraction of real, homogeneous materials cannot be below that of a vacuum, or 1.0000. Therefore, percentage changes in reference to indices of réfraction (such as 0.1% to 0.6%) cannot be employed literaily, they must be translated to reflect what was probably Intended, a différence in numerical value of the index of refraction above that of 1.000. It can be expected that a person skilled In the art of optlcal wavegulde design can successfully translate principles originally intended for near-1.46-lndex materials to apply to new materials with Indices far doser to 1.00.

INDEX OF REFRACTION OF FUSED SILICA AS A FONCTION OF ISOTOPIC PROPORTION OF Si-29 ISOTOPE ATOMS

As mentioned above, the index of refraction of silica will vary as the isotoplc percentage of Silicon-29 is varied from its normal (as found In nature) value of 4.67% atom/atom. In order to calculate the Index of refraction of silica for a given proportion of Si-29, the following formula Is used;

Index (silica) = SORT (1+ (1.131 ((percent Si-29 atom/atomy4.67%))) EXAMPLE TABLE FOR INDEX

OF REFRACTION OF FUSED SILICA GLASS:

ISOTOPIC PROPORTION

INDEX OF REFRACTION

OF FUSED SILICA GLASS

OF SI-29 ISOTOPE (atom/atom)

0.467%	(1/10	of	nature)	1.056
0.0467%	(1/100	of	nature)	1.0056
0.117%	(1/40	of	nature)	1.0140

(These figures disregard a small contribution by the Oxygen-17 content to the overall index of fused quartz glass.

The différence In the Index of refraction of fùsed silica with a Si-29 content 1/40 of nature’s value, and fused silica with a SI-29 content of 1/100 of nature’s value Is: 1.0140-1.0056 = 0.0084

This différence is approximately equal to the index of refraction différences employed by typical single-mode optical waveguides. It follows that an optical wavegulde can be constructed with a core région containing pure fùsed silica with an Isotopic proportion of sllicon of 0.117% (atom/atom), and a cladding région containing pure fused silica with an isotopic proportion ofsiiiconof 0.0467% (atom/atom). It will be apparent to a person skilled In the art of optical-waveguide design and manufacture that. since Independent and précisé control over the index of refraction of pure fused silica can be maintalned without the addition of any foreign dopants, an optical waveguide designer will hâve far greater control over the optical characteristics than existed previously.

The most obvious improvement that can be achieved is the construction of an optical waveguide employlng a material with an Index of refraction of the core of 1.02. The resulting velocity factor will be approximately (1/1.02), or 98% of c. Fibers of this characteristic, when depioyed, will dramatically reduce the data-iatency involved.

The second improvement, which Is somewhat less obvious, Is that since Si-29 Isotope atoms themselves represent a dopant, and because optical loss is a function of Rayleigh scattering caused by dopant atoms, a réduction in the concentration of Si-29 atoms by a factor of 40 (from the example above) may resuit in a 40x réduction in optical loss from the value of 0.19 decibels/kiiometer typically found in germaniadoped-core, silica-clad optical fibers. An optical loss of this magnitude, perhaps to 0.005 db/km, may vïrtually eliminate the need for optical amplifiers except on the very longest fiber links.

Such a large réduction In the inhérent loss of silica may uncover manufacturing flaws and limitations which will hâve to be corrected, and splidng-loss réduction research will take a new, greater urgency A splice which loses 0.2 decibel may hâve seemed reasonable when that was équivalent to 1.0 kilometer of fiberis loss (0.19 db/km), but that will become totally unacceptable when it is seen as the équivalent of 40 kilometers of fiberis loss (40 x 0.005db/km).

A third Improvement will very likely be a dramatic réduction in the chromatic dispersion of the silica itself, perhaps by as much as a factor of 40x with the 40x réduction of Si-29 isotope content described in the example above. Since the overall waveguide dispersion is a function of the sum of dispersions caused not merely by the material, but also by the geometry of the waveguide itseif, it is harder to forecast the overall improvements that could be achieved, but a réduction in the overall dispersion of 10 times In magnitude is plausible. Such an improvement would allow higher symbol-rates (often equal to bit-rates), and could dramatically reduce the need for complex dispersion-compensations that are currently used.

A fourth improvement will be a large broadenlng in width of useable optical frequencies, especially if ali optical losses are su bject to the same 40x réduction, commensurate with the 40x réduction In Si-29 proportion In the example described above. Currently the longest-haul data links are limited to wavelengths near 1540 nanometers, but useable régions mayextend to perhaps 500 to2000 nanometers, with limitations around 950 nm and 1.400 nm. Aoœssory optics and electronics needed to support such a wider bandwidth must become available, but a given installed, fiber may later be used to employ these new wavelength réglons.

EXAMPLES

1. An opticai waveguide could comprise a cladding layer formed of high-purity opticai glass, predominantly of silica or germania or both, and a core région formed of high-purity opticai glass, predominantly of silica or germania or both, where either or both of said glasses contain siiicon atoms of which less than 4.44% atom/atom are Siiicon-29 isotope atoms, or contain germanium atoms of which less than 7.41% atom/atom are germanium-73 isotope atoms, or both.

Each percentage could be a factor of 0.95 of the isotopic proportion of SI-29 and Ge-73 Isotope atoms normally found In terre s tri al samples of each element.

If this waveguide is constructed of silica least 10% of said oxygen atoms In the core or ciadding, respectively, could be Oxygen-18.

2. An opticai waveguide could comprise a cladding layer formed of high-purity opticai glass, predominantly of silica or germania or both, and a core région formed of high-purity opticai glass, predominantly of silica or germania or both, in which at least 50 mole percent of the oxygen in the core, and/or at least 50 percent of the oxygen In the cladding is Oxygen-18. Additionaliy, however, the core région and/or the cladding région could contain Oxygen-17 Isotope In a proportion of less than 5 atom % of the amount of Oxygen-18 isotope.

3. An opticai waveguide could comprise a cladding layer formed of high-purity opticai glass, predominantly of silica or germania or both, and a core région formed of high-purity opticai glass, predominantly of silica or germania or both, where wherein at least 70 atom % of the oxygen in the core, or cladding, or both, is Oxygen-18 isotope. Additionaily, however, the amount of Oxygen-17 atoms is less than 5 atom % of the total oxygen atom content.

4. Any of the opticai waveguides of Examples 1, 2 further Including a dopant in the core, or the cladding, or both. However, there is no requirement that the dopant In the core be Identical to the dopant In the cladding nor that they be or présent In Identical concentrations In the core and in the cladding. The dopant(s) may be germanium in naturel isotope proportion, germanium in any nonnatural isotope distribution, or Si-29 Isotope, or phosphorus, or a combination thereof.

5. In ail the opticai waveguides of Examples 1,2,3 the volume percent of the région containing SIO₂which Is depleted in Si-29, or the volume percent of the région containing GeO₂ which Is depleted in Ge73, could be less than 50%.

6. In the opticai waveguide of Example 1, at least 70 atom % of the oxygen in the core could be Oxygen-18, or at least 70 atom % of the oxygen in a région of the cladding adjacent to the core could be Oxygen-18, and the amount of Oxygen-17 could be less than 5 atom % of the total oxygen atom content in those régions, respectively.

7. In any of the optical waveguides of this invention the core could hâve a constant or gradient index of refraction.

8. In any of the opticai waveguides of this invention the fiber could be surrounded by an extemal iayer made of giass or plastic.

9. In any of the optical waveguides of this invention the giass material could include pure or doped germania giass, with the isotopic proportion of Ge-73 isotope reduced to at most 7.2 atom, In either or both of the core and cladding régions.

10. In any of the optical waveguides of this invention the isotopic proportion of0-18 could be ralsed In the core région, or the cladding région, or both, to at least 10 atom %, and to at most 100% of the total oxygen Isotope atoms présent.

11. In any of the optical waveguide of this invention the Si-29 isotope is présent as a dopant In the core, or in the cladding, or both.

12. In any of the optical waveguides of this invention the proportion of 0-17 Isotope could be above the naturel 00-17 Isotopic proportion of 0.038 atom % found on the earth.

13. In any of the optical waveguides this invention 0-17 can bedepleted in the core or cladding from the naturel 0-17 isotopic proportion of 0.038 atom % found on earth. Additionally there may be S1-29 Isotope in the core or cladding, or both, in naturel or non-naturel proportion.

14. The optical waveguides of this invention can be designed so that the différence in index of réfraction between the core and cladding régions is maintained, in full or in part, by a différence of isotopic proportions of Si-29 in silicon, or by a différence of Isotopic proportions of Ge-73 In germanium, of greater than 0.001 atom %.

15. In any optical waveguide of this invention -OH may be reduced by means of a deuterium rinse as has been described in Patent Abstracts of Japan, JP-A-60090845.

16. In any optical waveguide ofthis invention, the index of réfraction of the cladding may be reduced with a fluorine compound. In addition, the isotopic proportion of Si-30 or 0-18, or both, may be greater than that of nature In the core or the cladding or both.

17. This invention is also an opticaiiy-trensmÎssive material, primarily made of silica or germania or both, that has been isotopically modifïed to as to contaln less than 4.44 atom % of Si-29 atoms, or less than 7.41 atom % of Ge-73 atoms, or greater than 4.90 atom % of Si-29 atoms, or greater than 8.18 atom % of Ge-73 atoms.

18. In any of the optical waveguldes of this invention, the cladding or cladding layers may be doped with an isotope-modified sampling of germanium atoms, or the innermost cladding layer may be so modified, such that the isotope distribution of the germanium atoms présent has been reduced in Ge-73 isotope to at most 7.2 atom %. Preferably the amount of germanium dioxide is in the range of 0.005% 5 to 1 percent by weight; more preferably from about 0.1 % to about 0,5% by weight; and most preferably about 0.1% to about 0.3% by weight.

The instant has been described with référencé to particular embodiments. However, it should be obvious to those skilled in the art to which this invention pertains that other modifications and enhancements can be made without departing from the splrit and scope of the ciaims that follow.

Claims

Claims

1. An optical waveguîde comprising:

a) a cladding layer of first hlgh-purity optical glass; said first high-purity optical glass comprising one of silica, germania and a mixture of silica and germania; said first high purity optical glass having a first index of refraction;

b) a core région of second high-purity optical glass; said second high-purity optical glass comprising one of silica, germania and a mixture of silica and germania; said second high purity optical glass having a second Index of réfraction;

the atom percentage of Si-29 to ali other Si Isotopes in said silica being one of:

more than 0 and less than 4.44; and more than 4.90 and less than or equal to 100;

the atom percentage of Ge-73 to ail other Ge Isotopes in said germania being one of:

more than 0 and less than 7.41 ; and more than 8.18 and less than or equal to 100;
2. The waveguide of claim 1 In which the atom percentage of 0-17 to ail other O Isotopes in one of silica, germania and a mixture of silica and germania being one of:

more than 0 and less than 0.038; and more than 0.038 and less than or equal to 100.
3. The waveguide of claim 1 in which said first index of refraction Is higher than said second Index of refraction.
4. The waveguide of Claim 1, In which at least 10% of oxygen atoms In said silica are oxygen-18.
5. The waveguide of Claim 11n which at least 50 mole percent of oxygen In said core région is oxygen18 and less than 5 atom percent of oxygen in said core région Is oxygen-17.
6. The waveguide of Claim 1 in which at least 50 mole percent of oxygen in said cladding région is oxygen-18 and less than 5 atom percent of oxygen In said cladding région Is oxygen-17.
7. The waveguide of Claim 1 in which at least 70 atom percent of oxygen in said core région Is oxygen-18 and less than 5 atom percent of oxygen in said core région is oxygen-17.
8. The waveguide of Claim 1 In which at least 70 atom percent of oxygen in said cladding région is oxygen-18 and less than 5 atom percent of oxygen In said cladding région is oxygen-17.
9. The optical waveguide of Claiml in which said cladding région further comprises a dopant.
10. The optical waveguide of Claiml In which said core région further comprises a dopant.
11. The optical waveguide of Claim 8 or 9, In which said dopant is selected from the group consisting of germanium In natural isotope distribution, germanium In non-natural isotope distribution, silicon-29, phosphorus, silicon-29 and mixtures thereof.
12. The optical waveguide of Claim 1 In which the volume percent of the région containing silica containing less than 4.67 atom percent Si-29 Is less than 50.
13. The optical waveguide of Claim 1 in which the volume percent of the région containing germania containing less than 7.8 atom percent Ge-73, is less than 50.
14. The optical waveguide of Claim wherein said Indices of refraction change radially.
15. The optical waveguide of Claim 1 further comprising an extemal layer surrounding said cladding layer.
16. The optical waveguide of Claim 14 in which said extemal layer Is comprised of a substance selected from the group comprising glass and plastic.
17. The optical waveguide of claim 11n which hydroxyl concentration In said first and second high purity optical glasses Is reduced.
18. The optical waveguide of claim 16 In which réduction of hydroxyl has been accomplished by means of a deuterium rinse.
19. The optical waveguide as claimed In claim 11n which said first high-purity glass further comprises a fluorine containing compound whereby said first Index of refraction is further reduced.
20. The optical waveguide as claimed in claim 1 in which said cladding région further comprises 0.005 to 1 % by weight germanium dioxlde.
21. The optical waveguide as claimed In claim 11n which said cladding région furthercomprises 0.1 to 0.5 % by weight germanium dioxlde.
22. The optical waveguide as claimed In claim 1 in which said cladding région further comprises 0.1 to 0.3 % by weight germanium dioxlde.
23. An o ptically transmissive material comprising:

silica and germania and a mixture of silica and germania;

the atom percentage of SI-29 to ail other Si Isotopes In said silica being one of:

more than O and less than 4.44; and more than 4.90 and less than or equal to 100;

the atom percentage of Ge-73 to atl other Ge Isotopes tn said germanla being one of:

more than 0 and less than 7.41 ; and

5 more than 8.18 and less than or equal to 100;
24. An optically transmissive material as ctalmed in claim 23 In which the atom percentage of 0-17 to ail other O isotopes In one of silica, germanla and a mixture of silica and germanla being one of:

more than 0 and less than 0.038; and more than 0.038 and less than or equal to 100.