JP4540034B2 - Treatment of soot / preform with reducing agent - Google Patents

Abstract

A silica soot preform (12) is inserted into a furnace (30). The preform is then treated with heat and carbon monoxide gas (32) so as to reduce impurities that could effect the final product.

Description

Explanation of related applications

  Co-pending US Patent Application No. 60 / 295,052 (2001), both assigned to the assignee of the present applicant and filed under the name “Treatment of Soot Preforms with Reducing Agents”. Priority is claimed by reference to U.S. Patent Application No. 60 / 258,061 (filed on Dec. 22, 2000).

  The present invention relates generally to the manufacture of soot preforms, and in particular to the treatment of soot preforms with reducing agents.

In the manufacture of optical fiber (hereinafter referred to as “fiber”) and other products that can be manufactured from soot preforms, if the preform contains various impurities, the final product may cause various defects. is there. The effects of these defects are likely to appear in the attenuation of the signal propagated through the fiber and a significant decrease in the transmission of optical signals of specific wavelengths in the resulting glass article. When manufacturing optical fibers, it is desirable to minimize attenuation and thus eliminate defects that cause attenuation. The fact that impurities are contained in the fiber is a phenomenon that the attenuation of the fiber at a specific wavelength increases as the fiber is used for a long time. This effect of increasing attenuation is most likely to appear at wavelengths near at least 1200 nm.
US Pat. No. 5,534,994 US Patent Application No. 60/208342 US Patent Application No. 60/217967 US Patent Application No. 60/193080 US Patent Application No. 60 / 119,805 US Patent Application No. 60 / 123,861 US Patent Application No. 60 / 135,270 US Patent Application No. 60 / 159,076 US patent application Ser. No. 09 / 397,577 US patent application Ser. No. 09 / 397,573 US patent application Ser. No. 09 / 397,572 FOTP-78 FOTP-67 FOTP-20A TIA455-67 “Optical Fiber Telecommunications” edited by Stewart E. Miller and Alan G. Chynoweth, “Optical Fiber Telecommunications”, p. 214-218 FOTP-61

In order to minimize the possibility of the fiber having the above attenuation increase, among the methods that have been attempted in the past, there is a method of drying the preform using chlorine gas. Usually, the preform is placed in a furnace and then consolidated. A helium gas stream containing about 2% chlorine (Cl ₂ ) is introduced into the furnace, raising the furnace temperature to about 1000 ° C. and heating for up to about 2 hours. However, even if the soot preform was treated with chlorine gas, the increase in attenuation appearing in the fiber as described above could not be sufficiently suppressed.

  Another limitation in treating soot preforms with chlorine is that it is limited to glass compositions that are relatively inert to chlorination. Examples of glass compositions that are relatively inert to chlorination are silica glass or silica glass doped with germanium. However, not all silicate glass compositions will be inert to the chlorination described above. Examples of such silicate glass compositions that are not inert to chlorination include alumina, antimony, alkali oxides, boron oxides, phosphorus oxides, or alkaline earth oxides. There are compositions to include. It has been found that the above chlorination removes alumina, antimony, alkali or alkaline earth containing compounds from the preform.

  Furthermore, the above chlorination has also proved unsuitable for certain glass products that fluorinate after chlorination, because the preform is still in the preform even after the preform is doped with fluorine. This is because some chlorine remains due to chlorination. Chlorine remaining can affect the optical properties of the final glass product. For example, as a glass product which is not preferable when chlorine is mixed, there is a photomask plate for lithography. For example, the presence of chlorine in a lithographic photomask plate reduces the transmittance at various important wavelengths. One such wavelength is 157 nm. It has been found that the presence of about 75 ppm chlorine in the photomask plate reduces the transmission of light having a wavelength of about 157 nm by at least about 50%. For at least the above reasons, a new method for processing soot preforms is needed.

  Moreover, fluorine doped optical fibers showed increased attenuation at several wavelengths. The attenuation spectrum of an optical fiber doped with fluorine has absorption peaks at various wavelengths such as 1440 nm, 1546 nm, 1583 nm, and 1610 nm. At least the 1583 nm wavelength is an L-band transmission window, so an optical fiber with a fluorine-doped region should not have an absorption peak at the 1583 nm wavelength or any of the other wavelengths mentioned above There is.

  The present invention relates to soot preforms that can be used to produce optical products and the processing of the soot preforms. One embodiment of the present invention is a method of processing a soot preform. The method includes exposing at least one reducing agent to consume excess oxygen present in the furnace and exposing the soot preform to the furnace in a substantially chlorine-free atmosphere. including. A preferred reducing agent is carbon monoxide.

  An advantage of implementing the above method of the present invention is that it can be used to consolidate the soot preform in an environment that is clearly not an oxidizing environment. Furthermore, the method of the present invention makes it possible to produce an optical fiber that can remove excess oxygen from the preform and maintain good attenuation over a long period of time. A further advantage obtained by implementing the above method is that it makes it possible to increase the drawing speed of the fiber. One excellent specific application of the present invention is to use the above method in a process for producing non-zero dispersion shifted optical fibers.

  In a second method of practicing the invention, the preform is heated to a temperature below about 1000 ° C. to expose the preform to a reducing atmosphere. A suitable reducing atmosphere includes carbon monoxide. An advantage of carrying out the above method of the present invention is that a preform produced according to the above method can produce a chlorine-free glass. Chlorine-free glass means at least a preform that has not been exposed to chlorine during the manufacturing process. Chlorine-free glass has an excellent use as a photomask plate for lithography, and a photomask that does not contain chlorine can sufficiently transmit light having a wavelength of less than about 160 nm. Other advantages of implementing the above embodiments of the present invention include alumina, antimony, alkali oxides, alkaline earths that react with chlorine to evaporate or convert to crystalline chlorides. This embodiment can be used to process soot preforms containing oxides and other compounds. A further advantage of the above embodiment of the present invention is that it can be carried out at low temperatures, i.e. below about 1000 ° C.

The third method for carrying out the present invention is a method for manufacturing a preform for optical fiber. The method includes doping the soot body in an atmosphere containing carbon monoxide and at least one fluorine-containing compound having the general formula C _n F _{2n + 2} . “N” is preferably a positive integer. One advantage of incorporating the above-described method of the present invention into a process for manufacturing an optical fiber having a fluorine-containing region is that the fiber has an absorption peak at a wavelength of about 1440 nm, about 1546 nm, about 1583 nm, or about 1610 nm. It is not shown.

  Additional features and advantages of the invention will be set forth in the detailed description that follows and will be apparent to those skilled in the art from this specification, and may be found in the specification, including the description, claims, and accompanying drawings. It will be understood by carrying out the invention according to the description.

  The foregoing general description and the following detailed description are merely illustrative of the invention and are intended to provide an overview or framework for understanding the nature and characteristics of the invention as claimed. It was just. The accompanying drawings have been added to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. These drawings illustrate various embodiments of the invention and, together with the description, will serve to explain the principles and operation of the invention.

  Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. An exemplary embodiment of the method of the present invention in which a soot preform is treated with a reducing agent is shown in FIG.

The invention described herein relates to a soot preform and an innovative method of processing the soot preform. This preform is treated with a reducing agent. This preform is preferably treated with a compound containing CO or SO ₂ and no chlorine.

Soot Preform FIG. 1, represented by reference numeral 10, shows a soot preform 12 in a furnace 30. The soot preform 12 may use any known technique for forming the soot body. Such techniques include, for example, external vapor deposition (OVD), axial vapor deposition (VAD), modified chemical vapor deposition (MCVD), plasma enhanced chemical vapor deposition (PCVD), and others. However, the method is not limited to these methods. OVD, VAD, MCVD and PCVD are what are commonly referred to as chemical vapor deposition (CVD) methods. Preferably, soot is deposited on the starting member to form preform 12. The particle size of the soot particles deposited on the starting member is typically less than about 20 microns, preferably less than about 10 microns, more preferably about 0.1 to about 1.0 microns, and most preferably about 0.00. 1 to about 0.3 microns.

  The preform 12 includes a core region 14 and a cladding region 16. The preform 12 may optionally have a near cladding region (not shown). A central flow path 18 is provided in the core 14. A clad 16 is disposed around the core 14. The core 14 is typically made of silica or doped silica. The core 14 may optionally be doped with germanium to increase the refractive index of the core 14. The core 14 may optionally include a second dopant such as fluorine or, more preferably, may have an annularly doped region with fluorine. Other useful soot dopants include alkali metal oxides, alkaline earth oxides, transition metals, alumina, antimony oxide, boron oxide, gallium oxide, indium oxide, tin oxide, lead oxide, phosphorus oxide, oxide There are arsenic, bismuth oxide, tellurium oxide, selenium oxide, titanium oxide, and mixtures thereof. In the core 14, there may be a plurality of doped regions and undoped regions in the soot.

  The cladding 16 typically includes at least silica. The clad 16 should have a refractive index lower than that of the core 14. The materials used for the core 14 and the core 16 in the present invention are not limited to the above materials. The preform 12 may be made of any oxide glass.

As can be seen in FIG. 1, the preform 12 is provided with a handle 20 to which a standard ball joint handle 22 is fused. At the end of the core 14 opposite the handle 20, there is a plug 24, which may optionally have a capillary 26 attached thereto. The preform 12 is suspended in the furnace 30 by the handle 20.

Soot / Preform Processing Method
The first method of processing a treated soot preform at a temperature of at least about 1000 ° C. aims to reduce the attenuation at wavelengths above 1200 nm as the fiber is used longer. The increase in attenuation described above is a defect that appears after processing and is caused by the presence of both excess oxygen and at least one reduced germanium compound in the vicinity of each other in the drawn fiber. Because. The reduced germanium is a germanium element in the fiber and does not have four bonds with the oxygen element in the fiber. A list of reduced germanium compounds that may be present in the fiber includes Ge ⁺² , GeO ⁺¹ , Ge—Ge, but not all. The term “in the vicinity” means that excess oxygen is present within about 5 nm from the reduced germanium compound, preferably within about 1 nm from the reduced germanium compound, and most preferably within about 0.5 nm from the reduced germanium compound. It means to do. One major reason for the presence of excess oxygen in the drawn fiber is the consolidation of the soot preform in an environment where there is excess oxygen. Consolidation refers to heating the preform 12 to a temperature above about 800 ° C. and the soot preform, for example, but not limited to, various processes such as drying, doping, and sintering. Is defined as Excess oxygen is defined as the amount of oxygen present in the atmosphere beyond the amount that must be present to perform these consolidation steps. Some of the things that can cause excess oxygen include mixed oxygen, leakage into the consolidation furnace, and oxygen generated during the consolidation process. In this method of the invention, the soot preform 12 preferably includes at least one germanium doped region, preferably at least two germanium doped regions. Preform 12 is treated in furnace 30 using a reducing agent that removes any excess oxygen from the atmosphere in furnace 30. This reducing agent flows in the direction of arrow 32 to contact the preform 12 and removes excess oxygen. An example of a preferred reducing agent is CO. The reducing agent preferably reacts with excess oxygen to form a stable reaction product and consumes excess oxygen. For example, CO to form a CO ₂ reacts with excess oxygen. Another example of a suitable reducing agent is a reducing agent with reduced CO action, such as a CO / CO ₂ gas mixture.

  In addition to the reducing agent, an inert substance such as helium, argon, nitrogen or a mixture thereof may be optionally added to the atmosphere in the furnace. This inert material is not limited to the above. This atmosphere preferably contains substantially no chlorine-containing compound. If the treatment atmosphere includes carbon monoxide and an inert material and the preform 12 is not doped with fluorine, the concentration of carbon monoxide is at least about 100 ppm and not more than about 3000 ppm, more preferably at least about 200 ppm, most preferably Preferably it is about 300 to about 600 ppm.

  The reducing agent may be supplied to the furnace 30 during the drying operation of the preform 12 or during the sintering of the preform 12. When supplying the reducing agent to the furnace 30 during the drying operation, the preform 12 is heated to a drying temperature of about 1000 to about 1200 ° C. Preform 12 is preferably heated to about 1100 to about 1200 ° C. Preform 12 is maintained at its drying temperature for about 1 to about 6 hours. Preform 12 is preferably maintained at its drying temperature for about 4 hours. The atmosphere of the furnace 30 during the drying operation is preferably an atmosphere containing no halide. By carrying out the present invention, it is possible to manufacture a preform (also referred to as a blank) having a significantly reduced impurity concentration, and a fiber that does not exhibit an increase in attenuation as described above.

Optionally, the preform 12 can be doped with fluorine during drying or sintering. The fluorine doping is performed by heating the preform 12 to a doping temperature of about 1000 ° C. to about 1600 ° C. When the preform 12 is heated to an appropriate doping temperature, the preform 12 is exposed to a doping gas. The doping gas includes the following fluorine containing CF ₄ , SiF ₄ , C ₂ F ₆ , SF ₆ , F ₂ , C ₃ F ₈ , NF ₃ , ClF ₃ , BF ₃ , chlorofluorocarbon, and mixtures thereof. It preferably contains at least one of the gases. Preform 12 is exposed to the doping gas for about 1 to about 6 hours. It is preferred that the reducing agent be present in the doping atmosphere. As other optional steps, the soot / preform is performed either before the preform 12 is fluorine-doped or before the preform 12 is subjected to both the fluorine doping and the reducing agent treatment of the preform 12. Treatment of 12 with a halide-containing compound such as Cl ₂ or GeCl ₄ . It is preferable to supply the reducing agent to the furnace 30 after purging the halide-containing compound from the atmosphere of the furnace 30. The halide-containing gas is preferably a metal halide-containing compound or a dihalogenated compound. The temperature when the preform 12 is treated with the halide-containing compound is preferably about 800 ° C to about 1200 ° C, more preferably about 1000 ° C to about 1100 ° C. The halide treatment of preform 12 is preferably continued for about 1 hour to about 4 hours, more preferably about 2 hours. During halide processing, the atmosphere in the furnace 30 may contain an inert material such as helium, argon or nitrogen, as described above. After the preform 12 is halide treated, the atmosphere of the furnace 30 is preferably purged with an inert gas for at least about 5 minutes to about 2 hours.

  After the preform 12 is treated with the reducing agent, the central flow path 18 may be closed, and the preform 12 is sintered. One optional way to close the flow path 18 is to evacuate the central flow path 18. In order to sinter the preform 12, if necessary, the reducing agent is expelled from the furnace 30 and the furnace 30 is heated to a temperature of about 1200 to about 1600 ° C, more preferably about 1400 ° C or higher. You may add a reducing agent to the atmosphere in the furnace 30 during sintering as needed. It is preferred to sinter at least one inert gas, such as helium, argon, or other inert materials described above in the atmosphere. A suitable time for sintering the preform 12 is from about 0.5 hours to about 6 hours. In a preferred embodiment, the sintering time is about 4 to about 6 hours. However, the time required for sintering should be varied according to the sintering temperature, the size and density of the preform, and the chemical composition of the preform. Sintering may be performed in the same furnace as in the case of drying or in another furnace.

  If the core region 14 includes a dopant such as germanium, it is preferable to deposit germanium in excess to provide for side reactions that may occur between the reducing agent and the deposited germanium dioxide.

The whole is designated as reference numeral 40, and the sintered preform 42 can be drawn into a fiber 44 as shown in FIG. The sintered preform 42 is heated to a temperature of about 1800 ° C. or higher, and drawn into a fiber 44. Preferably, the sintered preform 42 is transferred to a drawing furnace 46 and the preform 42 is drawn into a fiber 44. A muffle furnace is preferably located at the outlet of the draw furnace 46. The fiber 44 is drawn by the tractor 50 and wound on the spool 52. The tractor 50 rotates in the direction of the arrow 54. Spool 52 rotates about axis A in the direction of arrow 56. A typical drawing speed is about 10 m / second or more, preferably about 20 m / second or more. In drawing the fiber, a tension of preferably about 75 to about 200 g, more preferably about 90 to about 150 g is applied. In another embodiment of this method, the reducing agent treatment is performed only during sintering. The soot is preferably a soot doped with something other than fluorine. In this embodiment, the reducing agent is supplied into the furnace 30 and the furnace is heated to a sintering temperature of about 1200 to about 1600 ° C., more preferably about 1400 ° C. or higher. The time for the sintering process is at least about 0.5 hours to about 6 hours. The concentration of the reducing agent in the sintering atmosphere is preferably about 300 to about 600 ppm. Optionally, the preform 12 may be exposed to an inert gas for about 0.5 hours at the end of the sintering process. In an embodiment added to the above-described method, the preform 12 as shown in FIG. 1 is a core cane preform, and the preform can be stretched into a core cane. is there. In this embodiment, additional soot is deposited on the preform 12 after the preform 12 is stretched into a core cane. The refractive index of this additional soot is preferably not higher than the refractive index of the core region having the highest refractive index. An example of a preferred material to deposit on the core cane is silica (SiO ₂ ). The silica can be doped with a refractive index increasing dopant or a refractive index decreasing dopant. Such a core cane coated with soot is also referred to as an overcladded preform or overcladded core cane.

  This overcladded preform is generally designated as reference numeral 60 and can be placed in a furnace and treated with a reducing agent as shown in FIG. The overcladded preform 62 includes a core cane 64 and at least one additional layer of soot 66 deposited on the cane 64. The cane 64 is preferably sintered into glass. The reducing agent is the same as described above. The furnace is heated in the aforementioned temperature range (about 800 to about 1200 ° C. or about 1200 to about 1600 ° C.) for the aforementioned time (about 0.5 to about 6 hours). The preform 62 is preferably exposed to a reducing agent during sintering.

  With the above-described method of the present invention, the possibility of excess oxygen being present in the furnace 30 during consolidation can be minimized. Thus, a fiber drawn from a preform treated as described above will not show the attenuation increase described above.

The above-described method of treating a soot preform using a reducing agent has excellent applications in the production of non-zero dispersion shifted optical fibers, and examples of such fibers include Corning (registered trademark). ) Submarine LEAF (Submarine LEAF) (registered trademark) (Corning Incorporated, Corning, NY). A non-zero dispersion shifted optical fiber is a fiber, preferably a segmented core fiber, having the properties listed in Table A (various definitions appear after the table).

  The following definitions are used to define the above properties.

Definitions The following definitions are the same as those commonly used in the art.

  “Refractive index profile” is the relationship between refractive index and waveguide fiber radius.

  A “segmented core” is a core that is divided into at least first and second waveguide fiber core portions or segments. Each portion or core is located at a specific radial distance, is substantially symmetrical about the waveguide centerline, and has an associated refractive index profile.

“Effective area” is expressed by the following formula:

Here, the range of integration is from 0 to infinity, and E is an electric field accompanying light propagating in the waveguide. The effective diameter, D _eff , can be defined by the following equation.

A _eff = π (D _eff / 2) ²
If that the effective area is large, the effective area of greater than about 60 [mu] m ² of fiber, more preferably the effective area is greater than about 65 .mu.m ² of fiber, and most preferably the effective area of the fiber means that greater than 70 [mu] m ² . It is possible to obtain fibers with an effective area greater than about 80-90 μm ² , and such fibers are preferred. “Relative Refractive Index Percent” is Δ% = 100 × (n _i ² −n _c ² ) / 2n _i ² , where _ni is the maximum refractive index in region i, unless otherwise noted, n _c, unless otherwise indicated, the average refractive index of the cladding region.

The term “α-profile” relates to the refractive index profile, expressed as Δ (b)%, where b is the radius, according to the following equation:
Δ (b)% = Δ (b ₀ ) (1- [| b−b ₀ |] / (b ₁ −b ₀ )] ^α ,
Here, b ₀ is a point where Δ (b)% is maximum, b ₁ is a point where Δ (b)% is zero, and b is in a range of b _i ≦ b ≦ b _f , where Where delta is as defined above, b _i is the start point of the α-profile, b _f is the end point of the α-profile, and α is a real exponent. The start and end points of the α-profile are selected and incorporated into the computer model. As used herein, if the α-profile is preceded by a step index file or other file type, the starting point of the α-profile is the α-profile and the step profile or other profile. The intersection of

  The above attenuation can be measured in accordance with US Pat. No. 6,047,028 issued on July 9, 1996, the specification of which is deemed to be incorporated herein. Other operating procedures that can be used to measure attenuation include Non-Patent Documents 1-4.

  The method of the invention can also be applied to fibers having at least one germanium doped region and at least one fluorine doped region. In this embodiment of the method, the preform 12 preferably includes at least one germanium doped region and a fluorine doped region formed during the soot deposition stage. Preform 12 preferably includes at least one fluorine doped region. This fluorine doped region may be formed during the fluorine doping process as described above, or may be formed during soot deposition. The fluorine doping process can be performed at a temperature of about 1100 ° C. or higher. The region doped with germanium and the region doped with fluorine may be completely different regions in the preform 12, or the regions overlap, and at least one region in the preform 12 is made of germanium. Both fluorine may be included, or the fluorine region and the germanium region may be combined.

  In this embodiment, the preform 12 is preferably treated with a reducing agent during sintering. However, in this embodiment of the invention, processing the preform 12 is not limited to only during sintering. Preform 12 may optionally be treated between the sintering and fluorine doping steps. The temperature of the reducing agent atmosphere is preferably at least about 1100 ° C, more preferably at least about 1300 ° C. As stated above, it is preferred that the reducing agent atmosphere be substantially free of any chlorine-containing compounds or elements.

In the case of a preform doped with fluorine, the concentration of the reducing agent in the atmosphere may be on the same order as the concentration of the fluorine doping agent in the doping atmosphere. As used herein, “same order” means that the concentration of the reducing agent in the reducing atmosphere is at least about 1/10 to less than about 3 times the concentration of fluorine in the fluorine doping atmosphere. is doing. For example, if the CF ₄ concentration in the doping atmosphere is about 1% by weight, the concentration of the CO reducing agent is preferably at least about 0.5% by weight, more preferably at least about 1.0% by weight. The present invention is not limited to the following examples.

One of the reasons that it is advantageous to treat a fluorine-doped preform with a reducing agent is that the fluorine doping process produces excess oxygen in the doping atmosphere. The following chemical reaction is involved in the incorporation of fluorine into the preform 12.

The reducing agent reacts with oxygen generated by the fluorine doping reaction to form CO ₂ . The amount of reducing agent in the atmosphere is preferably greater than the stoichiometric amount for oxygen produced during the fluorine doping reaction, so that excess reducing agent is present in excess excess that is present during the consolidation process. It becomes available for reaction with oxygen. Examples of possible sources of excess oxygen include mixed oxygen, oxygen leakage into the consolidation process, oxygen generated during the consolidation process, and in the consolidated gas. There are residual oxygen, but this is not all. By performing the reducing agent treatment as described above, all excess oxygen present in the furnace is consumed.

  Preform 12 may optionally be treated in a chlorine-containing atmosphere prior to fluorine doping or reduction treatment. The preferred chlorination process is the same as described above. In carrying out this method of the invention, no chlorination step is necessary.

  A fiber made by the above method and having a fluorine doped region does not exhibit the aforementioned phenomenon of increased attenuation as the fiber is used longer. This fiber is drawn from the preform 12 in the manner described above.

  The present invention can also be used to remove absorption peaks on the attenuation spectrum of fluorine doped fibers. An absorption peak is a region in the spectrum where the attenuation deviates from the baseline attenuation. This deviation typically increases the attenuation. FIG. 11 shows an attenuation spectrum of an optical fiber having a fluorine-doped region manufactured by a conventional method, and is designated as 110 as a whole. The spectrum measurement result is shown by line 112. In this analysis 110, the absorption peak 114 is at about 1440 nm, the second absorption peak 116 is at about 1546 nm, the third absorption peak 118 is at about 1580, and the fourth absorption peak 119 is at about 1610 nm. These absorption peaks, 114, 116, 118, and 119, are shown as deviations between the detected attenuation and the baseline of analysis 110 in FIG. 11 and each baseline is shown as a dotted line 120. , 122, 124 and 126. Absorption is the portion of attenuation that occurs when the optical energy of an optical signal is converted to heat.

  The attenuation spectrum of an optical fiber can be measured using various methods. One of the known techniques is Non-Patent Document 5. Pages 214 to 218 of Non-Patent Document 5 are incorporated herein by reference. An example of a type of apparatus that can be used to make the above measurements is a PK attenuation bench available from GN Nettest, Hopkinton, Massachusetts. One example of a suitable PK bench is PK-2500.

  Analysis of many optical fibers having portions doped with fluorine has found that the fibers have at least four different absorption peaks. The wavelength at which the absorption peak is present is nominally about 1440 nm (preferably at least about 1400 nm, more preferably at least about 1410 nm, most preferably at least about 1420 nm, and preferably about 1470 nm or less, more preferably about 1460 nm or less, most preferably About 1450 nm), nominally about 1546 nm (preferably at least about 1520 nm, more preferably at least about 1530 nm, most preferably at least about 1540 nm, and preferably about 1560 nm or less, more preferably about 1555 nm or less, most preferably about 1550 nm or less) , Nominally about 1583 (preferably at least about 1565 nm, more preferably at least about 1570 nm, most preferably at least about 1575 nm, and preferably about 1595 m or less, more preferably about 1590 nm or less, most preferably about 1585 nm or less), and nominally about 1610 nm (preferably at least about 1595 nm, more preferably at least about 1600 nm, most preferably at least about 1605 nm, and preferably about 1625 nm or less, More preferably, it is about 1620 nm or less, and most preferably about 1615 nm or less.

As found by the inventors, the cause of the above-described absorption peak is that during consolidation, the preform 12 is doped with a fluorine compound having the general formula C _n F _{2n + 2} (where n is a positive integer). It is to have done. The above-mentioned absorption peak appears in the spectral analysis because at least one of the following compounds is present in the optical fiber: CO, CO ₂ , COF ₂ , COClF, C _n F _{2n + 2} and mixtures thereof. It turns out that there is. The way in which the above compounds are formed is given as an example of the general formula for the case of CF ₄ (in this formula, the stoichiometric relationship is ignored):

The subscript “x” is a number from about 1 to about 4, preferably from about 1 to about 3.

It has further been found that the magnitude of this absorption peak increases for optical fibers having a delta% (Δ%) of about −0.35% or less, preferably −0.38% or less. Delta% is the relative refractive index percentage, and Δ% = 100 × (n _i ² −n _c ² ) / 2n _i ² , where n _i is the maximum refraction in region i unless otherwise noted. a rate, n _c, unless otherwise indicated, the average refractive index of the cladding region. The Delta% number noted above indicates the maximum drop in the region of the reduced refractive index of the fiber. An example of this will be described with reference to FIG. FIG. 14 is a refractive index profile of an optical fiber expressed in delta% as a function of radius. In the refractive index profile 140, the maximum delta% is less than about −0.4, indicated by 142. One aspect of the method for eliminating the above-described absorption peak is a method of making an optical fiber preform. The method includes doping the soot body in an atmosphere containing carbon monoxide and at least one fluorine-containing compound having the general formula C _n F _{2n + 2} (where “n” is a positive integer). . The doping atmosphere preferably contains at least about 5%, more preferably at least about 10%, even more preferably at least about 15%, and most preferably at least about 20% by volume of fluorine-containing compounds. It is also preferred that the fluorine-containing compound is about 50% by volume or less in the atmosphere. The soot body may be any of the above-described embodiments of the soot body 12 shown in FIG. An embodiment of this method of the invention is shown in FIG. 12 differs from FIG. 1 in that a gas 32 containing carbon monoxide and a fluorine-containing compound is flowed in the direction of the arrow 32 but only along the outside of the preform 12. There is no flow through. However, the present invention is not limited to the embodiment shown in FIG. Although not all of them, suitable fluorine-containing compounds include CF ₄ , C ₂ F ₆ or C ₃ F ₈ , and a more preferred compound is CF ₄ .

  Preferably, additional steps are added to the above method to expose the soot body to an atmosphere containing carbon monoxide and substantially free of halides. This exposure step is preferably performed prior to the doping step. Optionally, this exposure step may be performed after any chlorination step. It is preferred to purge the furnace between the chlorination step and this optional exposure step. Furthermore, this exposure step can be performed during the step of raising the soot preform from the final chlorination temperature to the fluorine doping temperature. The temperature and time of the chlorine process and the doping process are the same as described earlier in this specification. In one embodiment of this method of the invention, the atmosphere during the exposure step is a substantially chlorine-free atmosphere. The temperature of the preform during the exposure step is preferably at least about 1000 ° C, more preferably at least about 1100 ° C, and most preferably at least about 1200 ° C.

  When raising the temperature in the exposure step, the temperature in the furnace is preferably from above about 1000 ° C. to above about 1200 ° C., more preferably from at least about 1000 ° C. to at least about 1200 ° C., most preferably Is raised from at least about 1000 ° C to at least about 1300 ° C.

  The time for this exposure step may be between about 15 minutes and about 60 minutes, but suitable times for the exposure step are about 30 minutes, about 45 minutes, about 60 minutes. The furnace temperature is raised at a rate of about 2 ° C. to about 10 ° C. per minute. The heating rate during the exposure process can also be varied. Also, the temperature during the exposure process heating does not mean that the temperature must be raised continuously over the entire time of the exposure process. However, the present invention is not limited to raising the furnace temperature during the exposure process. The exposure process may be performed under isothermal conditions, or may be isothermal conditions as part of the exposure process.

  Preferably, the furnace atmosphere during the exposure step includes inert material and carbon monoxide, and the ratio of carbon monoxide to inert material is at least about 0.0012. More preferably, the ratio is about 0.48 or less. The inert material may be any inert gas, such as, but not limited to, argon, nitrogen, helium, and mixtures thereof. Furthermore, the concentration of carbon monoxide in the furnace during any of the exposure steps is preferably at least about 300 ppm, more preferably at least about 600 ppm, and most preferably at least about 1200 ppm. Further, the concentration is preferably up to about 4800 ppm, more preferably up to about 4000 ppm, and most preferably up to about 3000 ppm.

  The concentration of carbon monoxide in the furnace during the doping process may be similar to any of the concentrations described above for the exposure process. The concentration of furnace carbon monoxide during the doping process need not be the same as the furnace carbon monoxide concentration during the exposure process. Although the ratio of carbon monoxide to inert material described above for the exposure process may be performed at the ratio performed in the doping process, the ratio does not have to be the same as in the doping process.

  In this method, as described above, a step of sintering the preform may be further included. Alternatively, the present invention may include a step of sintering the preform in an atmosphere containing carbon monoxide. The preform is sintered in the same manner as previously disclosed, except that carbon monoxide is added to the sintering atmosphere. The concentration of carbon monoxide may be the same as described above for the exposure process, but the concentration of carbon monoxide in the sintering atmosphere is the same as the concentration of carbon monoxide in the furnace in the other processes. There is no need.

  The present invention further includes a step of drawing the preform into an optical fiber. When drawing the fiber from the furnace, the fiber preferably exits the furnace and then passes through a chamber filled with a gas having a lower thermal conductivity than helium. The temperature of the gas is preferably lower than the temperature of the drawn fiber. More preferably, the gas temperature is about room temperature or lower. Examples of gases with low thermal conductivity include nitrogen, argon, air, and mixtures thereof. The location of the chamber may be right next to the fiber exit from the draw furnace or may be significantly downstream from the fiber exit from the draw furnace. This chamber is also called a processing furnace or a low temperature extended muffle furnace (LEM). Further details regarding LEM can be found in a US patent application entitled “Methods and Apparatus for Forming Optical Fiber” filed around May 30, 2001. Patent applications are incorporated herein by reference.

  In one specific embodiment of the invention, the furnace is warmed up with the preform 12 already in the furnace for about 10 minutes, preferably about 5 minutes, more preferably about 3 minutes. The furnace preferably has two zones, namely zone 1 (also called consolidation zone) and zone 2 (also called sintering zone), each of which can be independently temperature controlled. Furnace zone 1 heats the temperature to about 1200 ° C or higher, preferably about 1225 ° C or higher. Furnace zone 2 is heated to about 1300 ° C. or higher, preferably about 1325 ° C. or higher, more preferably 1350 ° C. or higher, and most preferably about 1380 ° C. or higher. During warm-up, the furnace is flushed with inert gas at about 10 slpm (standard liters per minute = standard liters per minute), preferably about 15 slpm, more preferably about 20 slpm.

  Preferably, the preform is treated in a furnace zone 1 under a chlorine atmosphere of about 2.2% for at least about 30 minutes, more preferably for about 45 minutes, and most preferably for at least about 60 minutes. The furnace temperature is preferably kept the same during the warm-up process. During the chlorination process, about 20 slpm of helium and about 0.45 slpm of chlorine are passed through the furnace. When the chlorination process was completed, it was preferable to exhaust the atmosphere in the furnace from the furnace.

  The preform 12 is then exposed to an atmosphere of helium and carbon monoxide at the above temperature. This exposure may last between about 15 minutes and about 45 minutes. The exposure atmosphere includes helium and carbon monoxide, about 19.00 slpm helium, preferably about 19.24 slpm (more preferably about 19.48 slpm, most preferably at least about 19.76), and about 1. 00 slpm carbon monoxide (more preferably about 0.76 slpm, most preferably at least about 0.52 slpm, most preferably about 0.45 slpm or less) is introduced into the furnace. A 10% mixture of carbon monoxide and helium is preferred.

This embodiment further includes doping the preform 12 in an atmosphere containing helium, carbon monoxide and CF ₄ . The flow rate of helium to the furnace is about 14.00 slpm, preferably about 14.5 slpm, more preferably about 15.0 slpm, and most preferably about 15.76 slpm. The flow rate of CF ₄ to the furnace is at least about 2 slpm, preferably at least about 3 slpm, and most preferably at least about 4 slpm. The flow rate of carbon monoxide to the furnace is at least about 0.020 slpm, preferably at least about 0.024 slpm. The time for the doping step is at least about 30 minutes, preferably at least about 45 minutes, more preferably at least about 60 minutes, and most preferably at least about 90 minutes.

  This embodiment further includes degassing the doping atmosphere from the furnace. During the degassing step, the temperature in zone 2 of the furnace is increased to at least about 1400 ° C, preferably at least about 1420 ° C, more preferably at least about 1440 ° C. The furnace atmosphere during the deaeration process contains helium and carbon monoxide. Preferably at least about 19.0 slpm helium is injected into the furnace, more preferably at least about 19.5 slpm, and most preferably at least 19.76 slpm. The flow rate of carbon monoxide to the furnace is preferably at least about 0.020 slpm, more preferably at least about 0.024 slpm. The time for degassing the doping atmosphere is preferably at least about 10 minutes, more preferably at least about 20 minutes, and most preferably at least about 30 minutes.

  The preform is then sintered, but the preform is moved into the furnace zone 2 at a speed of at least about 4 mm / min. This speed may be increased to about 12 mm / min, but preferably about 8 mm / min. The temperature in zone 2 of the furnace is maintained at least about 1400 ° C, preferably at least about 1420 ° C, more preferably at least about 1440 ° C. The time for sintering the preform is at least about 120 minutes, preferably at least about 150 minutes, more preferably at least about 170 minutes. During the sintering process, an atmosphere of carbon monoxide and helium is fed into the furnace. Preferably at least about 19.0 slpm helium is injected into the furnace, more preferably at least about 19.5 slpm, and most preferably at least 19.76 slpm. The flow rate of carbon monoxide to the furnace is preferably at least about 0.020 slpm, more preferably at least about 0.024 slpm.

  Once the preform has been completely moved into furnace zone 2, the sintering process is complete and the preform is held in furnace zone 2 for a period of less than about 5 minutes, preferably about 5 minutes in a helium atmosphere. Less than 3 minutes, more preferably less than about 2 minutes, and most preferably less than about 1 minute. During this process, the helium supplied to the furnace is preferably 20 slpm.

  The last of this specific embodiment of the present invention is a finishing process. During the finishing process, the temperature in zone 2 of the furnace is reduced to less than about 1400 ° C, preferably less than about 1390 ° C, and most preferably about 1380 ° C. The atmosphere in the furnace during the finishing process preferably contains argon. The argon fed into the furnace is preferably about 10 slpm, more preferably more than about 15 slpm. Most preferably at least about 20 slpm. In the attenuation spectrum of the resulting fiber, it is preferred that the attenuation deviation shown in the wavelength range of about 1565 nm to about 1595 nm is about 0.012 dB / km or less. More preferably, this deviation is about 0.006 dB / km or less, more preferably about 0.003 dB / km or less, and most preferably the deviation is zero. This wavelength range is preferably in the range of about 1570 nm to about 1590 nm, most preferably about 1575 nm to about 1585 nm.

  In the attenuation spectrum of the resulting fiber, it is preferred that the attenuation deviation shown in the wavelength range of about 1400 nm to about 1470 nm is about 0.012 dB / km or less. More preferably, this deviation is about 0.006 dB / km or less, more preferably about 0.003 dB / km or less, and most preferably the deviation is zero. This wavelength range is preferably in the range of about 1410 nm to about 1460 nm, most preferably about 1420 nm to about 1450 nm.

  The attenuation spectrum of the resulting fiber preferably has an attenuation deviation of about 0.010 dB / km or less shown in the wavelength range of about 1520 nm to about 1560 nm. More preferably, this deviation is about 0.006 dB / km or less, more preferably about 0.003 dB / km or less, and most preferably the deviation is zero. This wavelength range is preferably from about 1530 nm to about 1555 nm, most preferably from about 1540 nm to about 1550 nm.

  The attenuation spectrum of the resulting fiber preferably has an attenuation deviation of about 0.012 dB / km or less shown in the wavelength range of about 1595 nm to about 1625 nm. More preferably, this deviation is about 0.006 dB / km or less, more preferably about 0.003 dB / km or less, and most preferably the deviation is zero. This wavelength range is preferably in the range of about 1660 nm to about 1620 nm, and most preferably about 1605 nm to about 1615 nm.

  Preferably, the spectrum of the drawn fiber does not show at least one of the aforementioned absorption peaks (nominal about 1440 nm, about 1543 nm, about 1583 nm and about 1610 nm). It is more preferred if the spectrum does not show at least two of the aforementioned absorption peaks, and most preferred if the spectrum does not show all of the aforementioned absorption peaks.

The maximum deviation can further be measured in the form of a root mean square (hereinafter referred to as RMS). The method for calculating the RMS follows the following equation:

Here, “n” is the number of sections within a given wavelength range, “λn” is the upper limit of wavelength, “λ1” is the lower limit of wavelength, and “attn _i ” is between n and 1 The attenuation exhibited by the fiber at any wavelength “i”. For example, for the 1583 absorption peak, the wavelength range includes from about 1565 nm to about 1595 nm. Preferably, the RMS is about 0.0090 or less, more preferably about 0.0088 or less, and most preferably about 0.0086 or less.

  The method described above can also be incorporated into a process for manufacturing an optical fiber. For example, the methods described above can be incorporated into a process for manufacturing large effective area optical fibers or dispersion controlled optical fibers. The dispersion control fiber preferably has at least one region containing a refractive index reducing dopant, and the maximum Δ% of the region is preferably about −0.35% or less, more preferably about −0. 38% or less, even more preferably about -0.40% or less, and most preferably about -0.42% or less. For further details of the dispersion control optical fiber, Patent Document 2 (filed on May 31, 2000), Patent Document 3 (filed on May 31, 2000), and Patent Document 4 (March 30, 2000) are described below. Which are incorporated herein by reference.

Treatment at a temperature below about 1000 ° C. In this method, the preform 12 suspended in the furnace 30 is heated to a temperature below about 1000 ° C. The heating temperature of the soot preform 12 is preferably less than about 800 ° C, more preferably about 25 ° C to about 600 ° C, and most preferably about 400 ° C or less.

As shown in FIG. 1, a reducing agent is supplied to the furnace 30. This reducing agent flows in the direction of arrow 32 and contacts the preform 12. The reducing agent is preferably a compound that reacts with an oxide of the form M _x O _y , where “M” is the periodic table with atomic numbers 21-30, 39-47, 57- 79, 89 to 107, and a mixture thereof, “O” is oxygen, “x” and “y” are integers of 1 or more. More preferably, “M” is at least one element selected from the group of elements consisting of Zr, Ni, Fe, Ti, V, Cr, Mn, Co, Cu, Zn, and mixtures thereof.

A preferred reducing agent is carbon monoxide. It is more preferable if this reducing agent reacts with the M _x O _y compound to obtain a reaction product containing at least one carbonyl complex (—CO). For example, when the preform 12 contains nickel oxide, the nickel oxide reacts with the carbon monoxide reducing agent to produce Ni (CO) ₄ . Most preferably, the carbonyl complex reaction product will volatilize at temperatures below about 200 ° C. under atmospheric pressure conditions. Reducing agents that can be used in place there is SO _2. In addition to the reducing agent, an inert gas such as helium, nitrogen, argon or a mixture thereof is preferably fed into the furnace 30 simultaneously with the reducing agent. The present invention is not limited to the inert gases described herein. Preferably, the preform 12 is exposed to the reducing agent for about 1 to about 4 hours.

  The soot preform 12 is a silicate body comprising at least one component selected from the group consisting of alkali metal oxides, alkaline earth oxides, aluminum oxide, antimony oxide, and mixtures thereof. Preferably there is. The preform 12 contains at least one element from the group of elements consisting of Sb, Al, B, Ga, In, Ti, Ge, Sn, Pb, P, As, Bi, Te, Se, and mixtures thereof. More preferably, the oxide is contained. Preform 12 may optionally be manufactured from a powder batch material for glass melting. Examples of suitable powder batch materials include silica, alumina, alkali oxides, and at least the other oxides described above.

Optionally, after the preform 12 is exposed to the reducing agent, the preform 12 is doped with fluorine. The fluorine doping is performed by heating the preform 12 to a doping temperature of about 1000 ° C. to about 1600 ° C. When the preform 12 is heated to an appropriate doping temperature, the preform 12 is exposed to a doping gas. The doping gas includes the following fluorine-containing gas selected from CF ₄ , SiF ₄ , C ₂ F ₆ , SF ₆ , F ₂ , C ₃ F ₈ , NF ₃ , ClF ₃ , chlorofluorocarbon, and mixtures thereof. It is preferable that at least one of them is included. Preform 12 is exposed to the doping gas for about 1 to about 6 hours.

The preform 12 is heated to a sintering temperature of about 1200 ° C. to about 1600 ° C. to sinter the soot preform 12 into glass. In the case of a fluorine doped preform, the preform can be sintered during or after fluorine doping. The preform 12 is maintained at its sintering temperature for about 1 to about 6 hours, preferably about 4 to about 6 hours. During sintering, the preform should be exposed to an inert atmosphere. Suitable inert atmospheres are those consisting of He, Ar, N ₂ or mixtures thereof. To form the tubular preform of FIGS. 4 and 5, the hole 18 was not closed as previously described.

  As shown in FIGS. 4 and 5, the sintered preform 12 has a length L in the longitudinal direction, and in the preform 12 with a cut, a cut is made along the length in the longitudinal direction. A cut 25 is formed. In the preferred embodiment, the preform 12 has only one cut, so that the cut 25 results in a single glass, a cross-section that resembles a very narrow “C” shape. A preform is obtained. The cut preform 12 is then flattened.

  In another embodiment, the preform 12 is cut into at least two longitudinal cuts and cut into at least two longitudinal pieces, but preferably the cuts are even in the circumferential direction of the preform 12. Enter a distance. When the inner circumference of the preform 12 is twice, three times, or four times the desired photomask width, the preform 12 is cut to have a size of 1/2, 1/3, or 1/4. Preferably it is a plurality of fragments. If the inner circumference of the preform 12 is substantially larger than the desired photomask blank width, it is a preferred method to cut into several sections.

  As shown in FIG. 6, the C-shaped piece 23 of the preform 12 can be flattened into a planar photomask blank member that is stretched under tension and further sagged, Similar to flattening a preform 12 having a single cut 25. Alternatively, the halved preform can be sagged directly in the furnace. Similarly, 1/3 of the preform 12 and 1/4 of the preform 12 can be directly sagged in the furnace 31. In order to flatten, it is preferable to place the preform section with the curved surface facing up.

  The consolidated glass preform 12 is preferably flattened with a cut after having a desired inner diameter and wall thickness. Such inner diameter and wall thickness are preferably adjusted by machining such as a core drill.

  In order to flatten the consolidated piece 23 of the preform 12, the piece 23 is heated and a force for deformation is applied to the heated piece 23. As shown in FIG. 6, as the step of flattening the piece 23, it is preferable to hang the piece 23 in a heated glasswork furnace 31 and attach a weight, for example, to apply a downward force to the piece 23. . The piece 23 is suspended from near the ceiling of the furnace 31 by being fastened to the hanging fastener 51. It is preferable to attach the platinum wire of the hanging fastener 51 in the vicinity of the cut 25 of the fragment 23. This platinum wire can be attached by making a hole in the piece 23 and passing the wire through. The hanger 51 and the platinum wire may be directly attached to the ceiling of the furnace 31 or may be indirectly attached to the furnace 31.

  Similarly, the downward force for flattening can be achieved by attaching a platinum wire to the piece 23 in the same way and placing a weight, preferably a silica weight, at the end of the cut opposite the piece 23 of the preform 12. Be suspended. A member 53 for applying a force provides a force for flattening the piece 23 and acts to extend the bending of the heated piece 23. The furnace 31 is preferably heated to at least about 1480 ° C. so that a member 53 for applying a force makes the piece 23 substantially flat. A suitable temperature for heating the piece 23 in the furnace 31 to extend the bend by this hanging extension is in the range of about 1480 to about 1580 ° C., preferably less than about 1600 ° C., for example about 1500 ° C.

  In addition to such a suspending method, further flattening the piece 23 heats the piece 23 to a sagging temperature and applies a deformation force to the heated piece 23. As shown in FIG. 7, the cut piece 23 that has been unwound by the suspension method shown in FIG. 6 can be further sagged in a glasswork furnace 31 to be further flattened. In this preferred process of sagging the piece 23 into a flat cut piece, the furnace 31 is heated to a sufficiently high temperature so that the weight of the raised part of the cut piece 23 acts as a sufficient deformation force. The incised tube is sagged to obtain a glass member having a flat surface.

  It is preferable to heat the furnace 31 to a softening temperature high enough to sag the cut piece 23 and keep it below the flow temperature, thereby allowing the glass to flow or thin while flattening the piece 23. It is substantially suppressed that it becomes. The sagging temperature is preferably in the range of about 1700 to about 1800 ° C, more preferably about 1720 to about 1760 ° C, and most preferably about 1730 ° C. In addition to applying a deformation force to the piece 23 by a method such as suspending and / or sagging and flattening the piece 23, it is also possible to apply a deformation force to the piece 23 heated in the furnace by pressing.

  When a pressing force is applied to the piece 23 in addition to the weight of the piece itself, the deformation temperature to be used can be lowered. If a pressing force is applied, the tube can be treated at a temperature of about 1550 to about 1650 ° C., and the pressing force can be applied to the piece 23 and dispersed using a flat planar pressure member. The surface of the piece 23 under pressure is covered with platinum foil for isolation from the glass preform. A flat planar member that can be used instead is a high-purity, high-density graphite slab. High purity high density graphite slabs can also be used as useful setters and furnace surface materials in the practice of the present invention.

In addition to flattening the piece 23, the piece 23 is preferably heated in a hydrogen-free heating environment, thereby preventing hydrogen from entering the glass and being trapped in the glass. All the H ₂ molecules are missed and degassed, resulting in a photomask blank containing no detectable hydrogen.

  Heating the piece 23 includes heating the piece 23 to the glass deformation softening point temperature, where the viscosity of the glass is reduced and the glass tube is deformed when a deformation force is applied.

  More detailed disclosure regarding the process of forming a photomask substrate from a consolidated preform can be found in the following US patent applications, which are incorporated herein by reference in their entirety: Shall. Patent Document 5 (filed on February 12, 1999), Patent Document 6 (filed on March 12, 1999), Patent Document 7 (filed on May 21, 1999), Patent Document 8 (October 1999) No. 12 (filed on Sep. 16, 1999), Patent Document 9 (filed Sep. 16, 1999), and Patent Document 11 (filed Sep. 16, 1999).

  The method of the present invention can also be incorporated into a process for making a photomask that does not contain chlorine. A chlorine-free photomask has the advantage that it can transmit at least about 90% of light with a wavelength of about 157 nm, more preferably about 100% of light with a wavelength of about 157 nm.

  The present invention will be described in more detail with reference to the following examples, and the purpose of these examples is to illustrate the present invention.

Reference Example A preform assembly consisting of a quartz tube and a quartz boat, in which the silica sand Iota-6 (Iota-6, UNIMIN, UNIMIN, N.C. And γ-Al ₂ O ₃ powder (available from Alfa Aesar, Ward Hill, Mass.) In a horizontal tube furnace at about 200 ° C. Heated for about 4 hours. During this heating, the preform was exposed to a CO reducing agent. CO gas was continuously supplied from one end of the boat at a flow rate of about 1 liter / min. The CO gas has a purity of about 99.7% and was obtained from BOC gas (BOC gas, Murray Hill, NJ).

The transition metal content of γ-Al ₂ O ₃ was measured before and after CO treatment. The transition metal content was measured by inductively coupled plasma mass spectrometry (ICP-MS) manufactured by Shiva Technologies (Syracuse, NY). The measurement results are shown in Table 1-1 below.

  The CO treatment reduced the concentration of each element to at least 1/3.

Example 1
The effect of treating the preform with a reducing agent to make a fiber was tested. Two preforms for manufacturing submarine LEAF (R) fiber (available from Corning Incorporated, Corning, NY) were treated in a chlorine-containing atmosphere before sintering. Treated with CO for about 2 hours. The CO concentration in the reducing atmosphere was changed. One preform was treated in an atmosphere containing about 300 ppm CO and the other was treated in an atmosphere containing about 600 ppm CO. These preforms were drawn into submarine LEAF (Submarine LEAF®) fibers. As shown in FIG. 8, the fiber sample was drawn at two different drawing speeds, and the tension was either about 90 g of low tension or about 150 g of high tension. The attenuation in each fiber drawn at a predetermined draw speed was measured at a wavelength of about 1550 nm for each 2 km long fiber sample. This 2 km long fiber was heated and held in an environmental chamber at a temperature of about 200 ° C. for about 20 hours. The environmental chamber used was a Yamato DKN600 model (available from VWR (West Chester, Pa.)). The change in attenuation was measured. A control fiber was also measured. This control fiber differs from the tested fiber in that the preform for drawing the control fiber was treated with Cl ₂ and not with CO. The attenuation of each sample was measured using a PK-8 spectral attenuation bench (available from Photon Kinetics, Beaverton, Oreg.), Following the operating procedure for the PK-8 bench. It was.

  The results for this sample are shown in FIG. As can be seen, the process of treating the fiber with a reducing agent reduces the induced attenuation. This is most noticeable in the case of a fiber drawn with low tension at a speed of at least about 10 m / sec. A significant reduction in inductive attenuation was observed for fibers made in accordance with the present invention.

Example 2
The effect of treating the fluorine-doped preform with the reducing agent described above was tested. Three preforms were treated with a chlorine-containing compound at a temperature of about 1000 ° C. Next, the temperature of the furnace was increased at 4 ° C./min to a temperature of about 1450 ° C. Each preform was treated in a fluorine-containing atmosphere for about 1 hour under the conditions shown in Table 3-1. As also noted in Table 3-1, the fluorine doping atmosphere for this test fiber also contained CO. This preform was drawn into a single-core fiber with a central core step index having an annular region doped with fluorine. Fiber samples were drawn with the same speed and the same tension. Attenuation for a 2 km long sample in each fiber was measured using a PK-2500 spectral attenuation bench (available from Photon Kinetics, Beaverton, Oreg.) At a wavelength of about 1550 nm. However, the operation procedure of the PK-2500 bench was followed. The fiber sample was held at a temperature of about 200 ° C. for about 20 hours. The change in attenuation was measured. A control fiber was also tested in the same manner. The difference between the control fiber and the test fiber is that the control fiber is dried with Cl ₂ and treated with CF ₄ instead of the reducing agent and fluorine-containing compound of the present invention described above. is there.

  For fibers made using the reducing agents disclosed in the present invention, the increase in attenuation was minimal or not observed at all. In contrast, a significant increase in attenuation was observed for the control fiber.

Example 3
The effect of treating the preform with CO during sintering was further investigated in terms of improving resistance to the hydrogen environment. Two preforms, dried in an atmosphere of Cl ₂ to helium and about 4 wt%. The preform was dried at a temperature of about 1000 ° C. to about 1150 ° C. for about 4 hours. One of the preforms was sintered at a temperature of about 1100 ° C. to about 1600 ° C. in an atmosphere containing at least about 200 ppm of CO and helium. The other preform was sintered under an inert atmosphere. These preforms were drawn into SMF-28 ™ optical fibers from Corning.

  A 2 km sample of each fiber was exposed to an atmosphere containing 1% hydrogen at room temperature and atmospheric pressure for about 144 hours. Fiber attenuation was measured for maximum attenuation observed before hydrogen exposure and 144 hours of exposure. Attenuation was measured according to Non-Patent Document 6 using a Photon Kinetics PK-6 attenuation bench.

  Fiber drawn from a preform treated with CO after the halide drying process is less susceptible to hydrogen induced attenuation compared to the control fiber, as seen in FIG. 9, and the attenuation of any fiber draw tension is There was little change.

Example 4
The effect of treating a fluorine-doped preform with a reducing agent was tested in this example. Each preform had a central core region doped with germanium, a fluorine doped region adjacent to the germanium central core, and a germanium annular region separated from the central core by at least a fluorine doped region. Since there is a moat depression due to the fluorine doped region, drawing this preform will result in a single mode fiber having a fluorine doped region with a delta% of about -0.5% or less.

The first preform was treated with carbon monoxide at a temperature of about 1000 ° C. prior to consolidation. The second preform was treated in a CO and Cl ₂ atmosphere at a temperature of about 1125 ° C., and the third preform was treated in a Cl ₂ atmosphere. Each preform was drawn at a drawing speed of at least about 10 m / sec to produce a single mode fiber.

The attenuation in each fiber drawn at a predetermined drawing speed was measured at a wavelength of about 1550 nm for a fiber sample of 2 km length. This 2 km long fiber was heated and held in an environmental chamber at a temperature of about 200 ° C. for about 20 hours. The environmental chamber used was a Yamato DKN600 model (available from VWR (West Chester, Pa.)). The change in attenuation was measured. A control fiber was also measured. The difference between this control fiber and the tested fiber is that the preform for drawing the control fiber is treated with Cl ₂ and not with CO. The attenuation of each sample was measured using a PK-8 spectral attenuation bench (available from Photon Kinetics, Beaverton, Oreg.), Following the operating procedure for the PK-8 bench. It was.

  The results for the sample are shown in FIG. As can be seen, the induced attenuation is significantly reduced by the process of treating the fiber with a reducing agent. Fibers treated with chlorine alone or at the same time as the reducing agent had higher induced attenuation. This phenomenon of increased inductive attenuation in fibers drawn from chlorinated preforms became more pronounced as the fiber drawing speed increased. A significant reduction in inductive attenuation was observed for fibers made in accordance with the present invention.

Example 5
In order to be used to form dispersion controlled optical fibers such as Vascule ™ fiber (available from Corning, Corning, NY), a process is used to remove the aforementioned absorption peaks. The effect of treating the reform with carbon monoxide was tested. Four preforms were tested. The process steps for forming each preform were all the same except as noted below. The control preform was not treated with carbon monoxide at all process steps. The process conditions for making the control preform are as follows.

1. The control preform was treated in a chlorine and helium atmosphere for about 90 minutes at a temperature of about 1150 ° C. with a chlorine flow rate of about 0.825 slpm to the furnace and a helium flow rate of about 20 slpm to the furnace. ;
2. The control preform was exposed to an inert atmosphere of helium, during which the furnace temperature was raised from about 1150 ° C. to about 1225 ° C., and the heating time was about 30 minutes;
3. This control preform was doped for about 120 minutes at a temperature of about 1225 ° C. in an atmosphere containing 15% CF ₄ and helium, but the flow rates of CF ₄ and helium to the furnace were ₃ slpm (CF ₄ ) and 17 slpm (helium), respectively. )Met;
4). A control preform was sintered at about 1490 ° C. Additional soot was deposited on the sintered preform. The soot-coated preform was sintered and then drawn into a fiber. The fiber was drawn at a tension of about 200 g and a drawing speed of about 9 m / sec.

  In addition to this control preform, three test preforms were made. The test preform differs in that carbon monoxide was used in the process steps during the process of forming the preform. In all process steps using carbon monoxide, the concentration of carbon monoxide was kept constant at 1200 ppm. The change in the use of carbon monoxide was made in the first preform in the presence of carbon monoxide in the furnace during the heating and doping steps. In the second preform, carbon monoxide was present in the furnace during the heating step, doping step and sintering. In the final test preform, carbon monoxide was present during the temperature raising step, the doping step, and the post-doping degassing step. The drying, temperature raising, and doping steps were as follows under the same conditions for each test preform.

1. The test preform was treated in a chlorine and helium atmosphere for about 90 minutes at a temperature of about 1150 ° C., with a chlorine flow to the furnace of about 1 slpm and a helium flow of about 40 slpm to the furnace;
2. The test preform was exposed to an inert atmosphere of helium and carbon dioxide, during which the furnace temperature was raised from about 1150 ° C. to about 1225 ° C., and the temperature rise time was about 30 minutes;
3. This test preform was doped for about 90 minutes at a temperature of about 1225 ° C. in an atmosphere containing 15% CF ₄ and helium, but the flow rates of CF ₄ and helium to the furnace were ₃ slpm (CF ₄ ) and 17 slpm (helium), respectively. Met;
4). The test preform was sintered at about 1490 ° C. Additional soot was deposited on each sintered preform. The soot-coated preform was sintered and then drawn into a fiber. The fiber was drawn at a tension of about 200 g and a drawing speed of about 9 m / sec.

  Each of the four preforms was drawn into an optical fiber. The attenuation spectra of each fiber in the wavelength range of about 1420 nm to about 1620 nm were measured and are shown in FIG. The fiber was tested on a Photon Kinetics (hereinafter “PK”) bench attenuation measurement device. A suitable device is a 2500 type fiber optic analysis system available from GN Nettest, Hopkinton, Massachusetts. That type of user manual is incorporated herein by reference. A method for measuring attenuation using the 2500 model is described in that manual. In this example, a step size of 1 nm was used. The length of each fiber sample used for the test was about 2 km, and each sample was used in the form of a loose coil.

  As shown in FIG. 13, in this control fiber, the aforementioned peaks at about 1440 nm, about 1543 nm, about 1580 nm, and about 1610 nm described above were observed (see line 132). Each absorption peak observed in the control fiber was greater than 0.012 dB / km. In each of the test preforms, none of the above-mentioned absorption peaks were observed in measurable magnitude (see lines 134, 136 and 138).

  It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Accordingly, it is intended that the present invention include modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

1 is a schematic cross-sectional view showing a preform placed in a furnace according to the present invention. FIG. 3 is a partial cross-sectional schematic view showing a state where an optical fiber is formed by drawing a consolidated preform. FIG. 2 is a partial cross-sectional schematic view of a soot-covered core cane placed in a furnace according to the present invention. 1 is a perspective view of a tubular consolidated preform made according to the present invention. FIG. 1 is a perspective view of a tubular solidified preform made according to the present invention with a cut in the longitudinal direction. FIG. FIG. 5 is an enlarged perspective view of a piece of tubular consolidated preform. It is a side view which shows the sagging of the piece of the solidified preform. FIG. 2 is a bar graph of induced attenuation at 1550 nm after exposure of a fiber made according to the present invention and a control fiber at 200 ° C. for 20 hours. A graph showing the relationship between hydrogen induced change in attenuation at 1530 nm and fiber drawing tension, fiber made from preform treated with CO after halide treatment, and no CO treatment after halide treatment. And a control fiber made from a preform. FIG. 4 is a graph of induced attenuation at 1550 nm after exposure of a fiber made according to the present invention and a control fiber at 200 ° C. for 20 hours. It is the spectrum which shows the relationship between attenuation | damping and a wavelength seen with the optical fiber which has the part doped with the fluorine. This optical fiber is manufactured by a conventional process. 1 is a schematic cross-sectional view showing a preform placed in a furnace by one method of the present invention. FIG. 4 is a spectral diagram showing the relationship between attenuation and wavelength for three preforms made by at least one method of the present invention and a control preform. A refractive index profile of an optical fiber representing delta% as a function of radius, showing the region where the refractive index is suppressed.

Claims (3)

Exposing the soot preform in an oven to an atmosphere containing carbon monoxide but substantially free of chlorine,
Doping the soot preform in an atmosphere comprising carbon monoxide and at least one fluorine-containing compound represented by the general formula C _n F _{2n + 2,} where n is a positive integer;
Having each process,
During the exposing step and the doping step, each atmosphere is brought to a temperature of at least 1100 ° C., and the concentration of carbon monoxide in the exposing step atmosphere is 1 / of the fluorine concentration in the doping step atmosphere. method of making a preform of an optical fiber, characterized in range near Rukoto three times from 10.   The method according to claim 1, further comprising the step of sintering the soot preform in an atmosphere containing carbon monoxide to form a sintered body. The method according to claim 1, wherein the atmosphere containing substantially no chlorine is heated in the exposing step.

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