RU2537523C1 - Radiation-resistant fibre-optic guide, method for production thereof and method of improving radiation resistance of fibre-optic guide (versions) - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: fibre-optic guide is obtained by chemical deposition of quartz glass from a mixture of starting gaseous reagents. The optic guide has a core of undoped quartz glass with low content of chlorine in the glass of the core due to considerable excess of oxygen O₂ relative to silicon tetrachloride SiCl₄ during manufacture.

EFFECT: providing high radiation resistance of an optic guide in the near infrared range by suppressing radiation-induced light absorption.

32 cl, 7 dwg

Description

Technical field

The invention relates to the field of fiber optical fibers that are resistant to nuclear and / or ionizing radiation, and is industrially applicable in fiber-optic communication systems intended for use under the conditions of exposure to the above radiation (inside and near nuclear energy facilities, objects with nuclear danger, on satellites, in armaments, in military and special equipment, etc.).

State of the art

It is known from the prior art that the most popular type of fiber optical fibers are silica glass fibers (i.e. having a core and cladding of doped or undoped silica glass) and being single-mode at a working wavelength of λ ₀ in the near infrared (IR) the range under which we will understand below the wavelength range of 0.78 ... 1.9 microns.

To date, the near-infrared range for optical communication uses the spectral range of ~ 1.29 ... 1.69 μm and primarily the most used wavelengths of 1.31 and 1.55 μm (see E. Desurvire "Optical Communication in 2025 ", 31 ^st European Conference on Optical Communication, ECOC-2005, Glasgow, UK, 25-29 September 2005, paper Mo 2.1.3). The most relevant modern applications of such optical fibers are optical communication systems, including the Internet (see EM Dianov “On the eve of the pet-era.” Uspekhi Fizicheskikh Nauk, vol. 183, No. 5, p.511-518 (2013)).

Preforms for such optical fibers are made by chemical deposition of silica glass from a mixture of the starting gaseous reagents. Technological processes implementing this method have been developed and are well known: MCVD, FCVD, VAD, OVD, PCVD and SPCVD. They are described, for example, in the following scientific articles: S.R. Nagel, J.B. MacChesney, K.L. Walker "An overview of the modified chemical vapor deposition (MCVD) process and performance" IEEE Journal of Quantum Electronics, vol. 18, No.4, pp.459-476 (1982); A.A. Malinin, A.S. Zlenko, U.G. Akhmetshin, S.L. Semjonov "Furnace chemical vapor deposition (FCVD) method for special optical fibers fabrication", Proc. SPIE, vol. 7934, Paper 793418 (2011); K. Okamoto, T. Edahiro, M. Nakahara "Transmission characteristics of VAD multimode optical fibers". Applied Optics, vol. 20, pp. 2314-2318 (1981); M.G. Blankenship, C.W. Deneka "The outside vapor deposition method of fabricating optical waveguide fibers", IEEE Transactions on Wicrowave Theory and Techniques, vol. 30, pp. 1406-1411 (1982); Th. Hunlich, H. Bauch, R.Th. Kersten, V. Paquet, G.F. Weidmann "Fiber perform fabrication using plasma technology", Journal of Optical Communication, vol. 4, pp. 122-129 (1987); E.M. Dianov, K.M. Golant, A.S. Kurkov, R.R. Khrapko, A.L. Tomashuk, "Low-hydrogen silicon oxynitride optical fibers prepared by SPCVD," Journal of Lightwave Technology, vol. 13, pp. 1471-1474 (1995). After that, fiber optic fibers are made from preforms by the known drawing method. In the drawing process, a protective coating is applied to the fiber. As a result, the fiber light guide consists of a core and a sheath based on quartz glass and a protective coating.

The most important characteristic of single-mode optical fibers is the cut-off wavelength of the first highest mode, λ _s . A fiber is single-mode at a wavelength of λ ₀ if λ ₀ > λ _c . (see F. Krahn, B. Sange, E.-G. Neumann, H. Schwierz, J. Streckert, F. Wilczewski "Cutoff wavelength of single-mode fibers: definition, measurement, and length and curvature dependence". Fiber and Integrated Optics, vol. 8, pp. 203-215 (1989)).

In a number of applications, optical fibers during operation are located in the fields of nuclear and / or ionizing radiation, or may be exposed to such radiation during operation. Nuclear radiation refers to fast neutrons, protons and beta radiation; Under ionizing radiation - gamma radiation, x-ray radiation, ultraviolet (UV) and visible radiation, which can fall on the fiber from the outside or propagate inside the fiber.

It is known that when using optical fibers under conditions of exposure to nuclear and / or ionizing radiation, a problem arises of increasing optical losses in the optical fiber until the transparency of the optical fiber is lost. This phenomenon is known as radiation-induced absorption (RNP) of light in a fiber (see B. Brichard, A. Fernandez Fernandez, H. Ooms, F. Berghmans, M. Decreton, A. Tomashuk, S. Klyamkin, M. Zabezhailov , I. Nikolin, V. Bogatyrjov, E. Hodgson. T. Kakuta, T. Shikama, T. Nishitani, A. Costley, G. Vayakis "Radiation-hardening techniques of dedicated optical fibers used in plasma diagnostic systems in ITER", Journal of Nuclear Materials, vol. 293-333, pp. 1456-1460 (2004)). The RNP effect is explained by the fact that the above radiation breaks chemical bonds in the glass fiber network of the fiber, because of which radiation color centers (RCOs) are formed in the glass network, which absorb light propagating through the fiber (see DL Griscom, KM Golant, AL Tomashuk , DV Pavlov, Yu.A. Tarabrin "Gamma-radiation resistance of aluminum-coated all-silica optical fibers fabricated using different types of silica in the core", Applied Physics Letters, vol. 69 (3), pp. 322-324 (1996)).

Fiber optic fibers for optical communication, widely used outside the fields of nuclear and / or ionizing radiation, have a core made of quartz glass doped with germanium oxide. At the same time, it is precisely on germanium atoms in the quartz glass network that a large number of RCOs arise, giving an unacceptably high RNP in the near infrared range, which is why such fiber optic fibers do not have the property of radiation resistance.

The prior art fiber optic fiber with increased radiation resistance, which instead of a core made of quartz glass doped with germanium oxide, has a core made of undoped quartz glass (see G. Tanaka, M. Watanabe, K. Yano "Characteristics of pure silica core single- mode fiber ", Fiber and Integrated Optics, vol. 7, pp. 47-56 (1987)). The increase in radiation resistance of this fiber is due to the exclusion of germanium atoms from the glass composition, which create a large number of RCOs.

The disadvantage of this fiber is the low radiation resistance due to three mechanisms of RNP affecting the propagation of the light signal in the near infrared range, the suppression of which would mean an increase in the radiation resistance of the fiber in this spectral region. These three RNP mechanisms are as follows:

- This is primarily RNP caused by RCOs bound to chlorine atoms that enter the network of nominally undoped silica glass during the synthesis of glass blanks from a mixture of the initial gaseous reagents, usually containing molecular oxygen O ₂ and silicon tetrachloride SiCl ₄ . This RNP increases with increasing chlorine content in the glass fiber. At the same time, it reaches a maximum in the UV range and decreases monotonically with increasing wavelength, manifesting itself in the near IR range (see S. Girard, C. Marcandella, A. Alessi, A. Boukenter, Y. Ouerdane, N. Richard , P. Paillet, M. Gaillardin, M. Raine “Transient radiation responses of optical Fibers: influence of MCVD process parameters”, IEEE Transactions on Nuclear Science, vol. 59, No. 6, pp. 2894-2901 (2012)). During irradiation of a fiber with nuclear and / or ionizing radiation, this RNP increases monotonically with increasing dose (see DL Griscom, KM Golant, AL Tomashuk, DV Pavlov, Yu.A. Tarabrin "Gamma-radiation resistance of aluminum-coated all-silica optical fibers fabricated using different types of silica in the core ", Applied Physics Letters, vol. 69 (3), pp. 322-324 (1996)). In the future, this RNP will be called "RNP-1";

- the second mechanism is RNP, not associated with chlorine atoms. It also has a maximum in the visible or UV spectrum and monotonically decreases with increasing wavelength. At high dose rates of ionizing radiation, this RNP depends on the dose nonmonotonically: it increases sharply at the beginning of irradiation and then decreases with increasing dose (see AL Tomashuk, KM Golant "Radiation-resistant and radiation-sensitive silica optical fibers", Proceeding of SPIE Vol. 4083, pp. 188-201 (2000)). The nature of the RCOs responsible for this RNP is not known for certain and is still not theoretically explained. In the future, this RNP will be called "RNP-2";

- the third mechanism is RNP, which reaches a maximum at a wavelength of about 1.9 μm and monotonously decreases with decreasing wavelength (see E. Reginer, I. Flammer, S. Girard, F. Gooijer, F. Actten, G. Kuyt "Low-dose radiation-induced attenuation at infrared wavelengths for P-doped, Ge-doped and pure silica-core optical fibers", IEEE Transactions on Nuclear Science, vol. 54, No. 4, pp. 1115-1119 (2007)) . The nature of the RCOs responsible for this RNP is also not known for certain. In the future, this RNP will be called "RNP-3."

One of the promising areas of development of radiation-resistant fiber optical fibers is the manufacture of their components from quartz glass doped with fluorine.

As a fairly close analogue of the proposed fiber, a radiation-resistant fiber is known with a core made of undoped quartz glass KS-4B and a sheath made of quartz glass doped with fluorine (see VA Bogatyrjov, II Cheremisin, EM Dianov, KM Golant, AL Tomashuk "Super- high-strength metal-coated low-hydroxyl low-chlorine all-silica optical fibers ", IEEE Transactions on Nuclear Science, vol. 43, No.3, pp.1057-1060 (1996)). The high radiation resistance of the fiber is due to the low content of chlorine impurities (0.002 weight percent), due to which RNP-1 is suppressed. Also suppressed RNP-3. Quartz glass KS-4V is made in the form of volumetric blocks, and a rod is drawn from the block for the manufacture of a fiber fiber preform, onto which a reflective cladding is made of quartz glass doped with fluorine.

The disadvantage of this close analogue is that this fiber is not single-mode in the near infrared range, since it is almost impossible to carve a sufficiently thin uniform rod (with a diameter of less than 1 mm) for use as the core of a single-mode fiber preform. Another disadvantage is the fact that RNP-2 is present in this fiber.

From foreign patent publications known radiation-resistant fiber optic fiber based on silica glass, as well as a method for its manufacture, including the manufacture of a workpiece containing a core and a sheath, which are synthesized by the chemical deposition of silica glass from a mixture of the source gaseous reagents, and the subsequent drawing of the fiber optic fiber from blanks (see US patent 7440673 B2 "RADIATION RESISTANT SINGLE-MODE OPTICAL FIBER AND METHOD OF MANUFACTURING THEREOF", publ. 21.10.2008, IPC G02B 6/00). In the manufacture of a preform, the quartz glass of the core is doped with fluorine, so that its refractive index becomes 0.1 ... 0.3% less than the refractive index of undoped quartz glass, and the quartz glass of the shell is doped with even more fluorine, so that its refractive index becomes lower than the refractive index core by 0.3 ... 0.5%. The bottom line is that when doping quartz glass with fluorine during the synthesis of glass preforms, the entry of chlorine atoms into the glass network is suppressed (atoms of more chemically active fluorine replace chlorine atoms). Therefore, due to the small amount of chlorine in the core glass grid, RNP-1 is minimized. In addition, due to the presence of fluorine in the glass, RNP-3 is suppressed in the fiber. Therefore, the fiber has a high radiation resistance.

The disadvantage of this method of manufacturing a radiation-resistant fiber is the difficulty of its practical implementation, since the preparation of a preform requires the sequential application of two technologies at once: first, VAD technology for the synthesis of core glass, and then OVD technology for the synthesis of cladding glass. In addition, the disadvantage of this radiation-resistant fiber and the method of its manufacture is the fact that the RNP-2 in the fiber is not completely suppressed.

Closer to the method of manufacturing the inventive fiber can be recognized as a method of manufacturing a radiation-resistant fiber based on quartz glass doped with fluorine, presented in the descriptions of US patent 7689093 B2 "Fluorine-Doped Optical Fiber", publ. 03/30/2010, IPC G02B 6/00, G02B 6/02 and 7526177 B2 "Fluorine-Doped Optical Fiber", publ. 04/28/2009, IPC G02B 6/00; G02B 6/036, based on the general priority application FR 20060006058 of 04.07.2006. The method includes the manufacture of a preform containing a core and a cladding, which are synthesized in the framework of a single (and not double, as in the previous analogue) technological process for the chemical deposition of silica glass from a mixture of initial gaseous reagents, and drawing a fiber optic fiber from the preform. In this method, the core and the shell are also synthesized from fluorine-doped silica glass, wherein the core refractive index is more than 1.5 · 10 ^-3 lower than the refractive index of undoped quartz glass, the shell refractive index is more than 4.5 · 10 ^-3 lower the refractive index of undoped quartz glass, and the difference in the refractive indices of the core and shell is not less than 3 · 10 ^-3 . In this case, the mixture of the initial gaseous reagents contains reagents that allow the synthesis of quartz glass doped with fluorine. A preferred embodiment of the method is the chemical deposition of silica glass on the inner surface of a silica glass support pipe (PCVD process). In this case, the glass of the support tube becomes the outer shell of the fiber, while the inner shell is formed by chemically deposited glass. The essence of the method is the same as that of the previous analogue: when doping quartz glass with fluorine during the synthesis of glass billets, the entry of chlorine atoms into the glass grid is suppressed (atoms of more chemically active fluorine replace chlorine atoms), therefore, due to the small amount of chlorine in the core glass grid RNP-1 is minimized. In addition, due to the presence of fluorine in the glass, RNP-3 is suppressed in the fiber. As a result, the fiber light guide has increased radiation resistance.

A drawback of this close analogue is the fact that an admixture of fluorine in the core decreases its refractive index, which, due to the presence of an outer cladding, deteriorates the light-guiding properties of such a fiber as compared with a fiber whose core is made of undoped quartz glass. In particular, in such a fiber waveguide with a fluorine-doped silica glass core, optical losses can occur when it is bent. Another disadvantage is the fact that RNP-2 in a fiber with a core and a sheath made of quartz glass doped with fluorine is not completely suppressed.

As for the analogues of the method for increasing the radiation resistance of a fiber waveguide, from foreign patent publications there is known a method for increasing the radiation resistance of a silica glass fiber with an undoped quartz glass core, which consists in saturating the fiber with molecular hydrogen and irradiating with gamma radiation (patent USA 5267343 "Enhanced radiation resistant fiber optics", published on November 30, 1993, IPC G02B 6/00, G02B 6/02, C03C 25/60; C03C 25/62). The essence of the method is that in the process of gamma irradiation, hydrogen atoms enter the glass network at the site of the RCC precursors (irregular chemical bonds in the glass network at the site of which the RCC under the influence of radiation) and thereby suppress them. After gamma-irradiation of a fiber in the presence of hydrogen molecules, the glass grid no longer contains RCE precursors responsible for RNP-1 and RNP-2. Therefore, the fiber optic fiber acquires the property of increased radiation resistance.

The disadvantage of this method is that its use does not suppress the precursors of RNP-3. Another disadvantage of this method is the fact that when it is used due to the absorption of OH hydroxyl groups arising in the glass fiber network of the fiber due to the presence of hydrogen, optical losses in the near infrared range increase. Therefore, this method is effective for fiber optical fibers that are used only in the visible spectral range, but is not applicable for fiber optical fibers intended for operation in the near infrared range.

A more close method is also known for increasing the radiation resistance of a fiber containing a core and a cladding based on silica glass, as well as a sealed protective coating deposited on top of the cladding, and containing hydrogen and / or deuterium molecules in high concentrations, providing increased radiation resistance of the fiber, also a method of manufacturing such a fiber, including pulling the fiber from the workpiece, applying to it in the process of drawing a sealed coating and subsequent placement of hydrogen and / or deuterium in a gaseous atmosphere at high pressures and temperatures (see RF patent No. 2222032 "FIBER FIBER (OPTIONS) AND METHOD FOR ITS PREPARATION", published on April 27, 2002, IPC G02B 6/16, C03C 25 / 60, C03B 37/01). The essence of the invention is that the hydrogen molecules of H ₂ and / or deuterium D ₂ diffuse through a sealed coating into the glass of the optical fiber and for a long time remain in the glass of the optical fiber after removing it from the gas atmosphere. When an optical fiber is exposed to ionizing and / or nuclear radiation, hydrogen and / or deuterium atoms enter the glass fiber network in places torn by radiation of chemical bonds, thereby eliminating the RCO responsible for RNP-1 and RNP-2. Therefore, the fiber optic fiber has the property of increased radiation resistance.

The disadvantage of this method of increasing radiation resistance is that in the fiber is not reduced RNP-3. Another disadvantage of this method is the fact that due to the absorption of hydroxyl groups of OH and / or OD groups arising from hydrogen molecules H ₂ and deuterium D ₂ in the glass network under the influence of radiation, the optical losses in the optical fiber in the near-infrared region increase range. Therefore, this method is effective for increasing the radiation resistance of fiber optical fibers used only in the visible spectral range, but is not applicable for ensuring the radiation resistance of fiber optical fibers intended for operation in the near infrared range.

Disclosure of invention

The objective of the invention was that the radiation-resistant fiber, the method of its manufacture and the variants of the method of increasing the radiation resistance of the fiber, overcome these disadvantages of the known close analogues, namely, they significantly suppressed all three mechanisms of RNP and thereby provide increased radiation resistance optical fiber in the near infrared range. It was also necessary to eliminate the other above-mentioned disadvantages of analogues.

The technical effect is achieved in that one of the features of the proposed radiation-resistant single-mode fiber waveguide with an undoped silica glass core obtained with a significant excess of oxygen is the low chlorine content in the core glass. The authors experimentally established that in the proposed fiber all three RNP mechanisms were suppressed and therefore higher radiation resistance was ensured than that of the analog fiber. In this case, the creation of a significant excess of the molar flow rate of O ₂ over SiCl ₄ in the mixture of the initial gaseous reactants during the synthesis of the glass core is the only possible way to manufacture such a fiber waveguide. Another significant drawback of the analogue optical fiber is eliminated: the proposed radiation-resistant optical fiber is single-mode in the near infrared range, but the analogous optical fiber is not.

The authors of the present invention experimentally established that by optimizing the costs of the reagents in the synthesis of glass of the core of the workpiece by the method of chemical deposition of silica glass from a mixture of the initial gaseous reagents, namely by providing a significant excess of the molar flow rate of molecular oxygen O ₂ over the molar flow rate of silicon tetrachloride SiCl ₄ , it is possible to create a single-mode blank an optical fiber with a core of undoped silica glass, in which the chlorine content is reduced, and therefore fiber optic fiber suppressed RNP-1. It has also been experimentally established that all other RNP mechanisms (RNP-2 and RNP-3) are also suppressed. Thus, with this manufacturing method, the high radiation resistance of the fiber is ensured, and due to the fact that the core is made of undoped quartz glass, i.e. does not contain fluorine, there is no deterioration in the light-guiding properties of the fiber, in contrast to the manufacturing methods, which are analogues.

The authors experimentally established that the method of increasing the radiation resistance of the fiber, which consists in the fact that the synthesis of the core of the fiber preform provides a significant excess molar flow of molecular oxygen O ₂ over the molar flow of silicon tetrachloride SiCl ₄ , can be applied to fibers with an undoped core or core, doped with fluorine. Unlike analogs, along with the suppression of all known RNP mechanisms in the near infrared range and therefore providing higher radiation resistance, the proposed method of increasing radiation resistance eliminates another drawback of analogues, namely, when applying the proposed method, optical losses in the fiber in near infrared range caused by absorption of OH and OD groups in the glass network.

The authors also found that radiation resistance also depends on the technological modes of synthesis of the blank shell. To increase the radiation resistance of the fiber, it is necessary to synthesize the cladding region immediately adjacent to the core by the usual (“direct”) deposition of quartz glass doped with fluorine when SiCl ₄ , SiF ₄ and O _{2 are} fed into the initial mixture. In this case, a small value of the difference in the refractive indices of the core and shell is achieved (not more than 0.006). Therefore, to improve the light-guiding properties of the fiber in the peripheral region of the cladding, it is necessary to obtain a very small refractive index (the difference in refractive indices with the core is more than 0.007). For this, this region of the shell should be precipitated by impregnation of the porous SiO ₂ layer with silicon tetrafluoride. It is important that in the inventive method of manufacturing a fiber optic fiber to ensure high radiation resistance does not require the introduction of fluorine into the core and therefore, accordingly, the light-transmitting properties of the inventive fiber are not degraded.

Thus, according to the first independent claim (aspect) of the invention, there is provided a radiation-resistant fiber waveguide comprising a silica glass core and cladding and a protective coating, wherein the core is made of undoped silica glass, the cut-off wavelength of the first highest mode of said fiber does not exceed 1.7 microns, and the chlorine content in its core does not exceed 0.01 weight percent.

In addition, in the radiation-resistant fiber, according to the specified aspect, the chlorine content in the core exceeds 1 · 10 ^-5 weight percent, and the amplitude of the absorption band of hydroxyl OH groups at a wavelength of 1.38 μm in the optical loss spectrum of the fiber does not exceed 9 dB / km The sheath of the inventive fiber may consist of an outer and inner sheath or only the inner sheath. In this case, the inner shell can be made of quartz glass doped with fluorine and contain an annular region adjacent to the core, the refractive index of which is less than the refractive index of the core by no more than 0.006. It is advisable that in this case there is an annular region in the inner shell, the refractive index of which is less than the core refractive index by more than 0.007. The radiation-resistant fiber can be microstructured, or photonic crystal, or birefringent.

According to a second independent aspect of the invention, there is provided a method for manufacturing a radiation-resistant silica glass fiber based on a silica glass, the method comprising fabricating a preform containing a core and a sheath using a single process of chemical deposition of silica glass from a mixture of the starting gaseous reagents, and then drawing the fiber from the preform, glass core lead from a mixture of silicon tetrachloride and molecular oxygen and provide excess molar molecular isloroda above molar flow rate of silicon tetrachloride was at least 75 times.

In this case, it is possible to carry out the chemical deposition of quartz glass on the inner surface of the support pipe from undoped or fluorine-doped silica glass, which is the outer sheath of the fiber, which, in particular, is removed before the fiber is drawn. It is advisable to synthesize the shell from a mixture of the initial gaseous reagents containing silicon tetrachloride, silicon tetrafluoride and molecular oxygen, while the region of the shell adjacent to the core, it is advisable to synthesize by direct deposition of quartz glass doped with fluorine, and the peripheral region of the shell by impregnation of the porous layer silica glass with silicon tetrafluoride.

According to a third independent aspect of the invention, there is provided a method for increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is prepared by chemical deposition of silica glass from a mixture of the starting gaseous reactants containing silicon tetrachloride and molecular oxygen, moreover, mixtures provide an excess of the molar flow rate of molecular oxygen over the molar flow rate of silicon tetrachloride in enshey least 75 times.

In this case, it is advisable to synthesize the blank shell by chemical precipitation of silica glass from a mixture of initial gaseous reactants containing, for example, silicon tetrachloride, silicon tetrafluoride and molecular oxygen. In this case, the cladding region adjacent to the core is useful for synthesizing by direct deposition of fluorine-doped silica glass and the peripheral region by impregnation of the porous silica glass layer with silicon tetrafluoride. It is also advisable to carry out the deposition of the core and shell on the inner surface of the support pipe from undoped or fluorine-doped silica glass.

According to a fourth independent aspect of the invention, there is provided a method for increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is made by chemically precipitating silica glass from a mixture of starting gaseous reactants containing silicon tetrachloride, molecular oxygen and silicon tetrafluoride, moreover, in said mixture, the molar flow rate of molecular oxygen is exceeded over the molar flow rate t trahlorida silicon is at least 70 times.

In this case, it is also advisable to synthesize the blank shell by chemical precipitation of silica glass from a mixture of initial gaseous reactants containing, for example, silicon tetrachloride, silicon tetrafluoride and molecular oxygen. In this case, the cladding region adjacent to the core is useful for synthesizing by direct deposition of fluorine-doped silica glass and the peripheral region by impregnation of the porous silica glass layer with silicon tetrafluoride. It is also advisable to carry out the deposition of the core and shell on the inner surface of the support pipe from undoped or fluorine-doped silica glass. It is also possible to add Freon 113 to the mixture of the starting gaseous reagents during core synthesis.

According to a fifth independent aspect of the invention, there is provided a method for increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is prepared by chemical deposition of silica glass from a mixture of starting gaseous reagents containing silicon tetrachloride, molecular oxygen and freon 113, moreover, in the specified mixture provide an excess of the molar flow rate of molecular oxygen over the molar flow rate of tetrachloride cream Ia is at least 70 times.

In this case, it is advisable to synthesize the blank shell by chemical precipitation of silica glass from a mixture of initial gaseous reactants containing, for example, silicon tetrachloride, silicon tetrafluoride and molecular oxygen. In this case, the cladding region adjacent to the core is useful for synthesizing by direct deposition of fluorine-doped silica glass and the peripheral region by impregnation of the porous silica glass layer with silicon tetrafluoride. It is also advisable to carry out the deposition of the core and shell on the inner surface of the support pipe from undoped or fluorine-doped silica glass. It is also possible to add silicon tetrafluoride to the mixture of the starting gaseous reagents during core synthesis.

Thus, the claimed radiation-resistant fiber, the method of its manufacture and variants of the method of increasing the radiation resistance of the fiber overcome the disadvantages of the identified analogues.

Brief Description of the Drawings

Figure 1 presents a longitudinal section of the inventive fiber.

Figure 2 schematically shows the profiles of the refractive index of the optical fibers described in the embodiment, including the inventive optical fiber.

Figure 3 presents the spectrum of the initial (measured before gamma radiation) optical loss in the inventive radiation-resistant fiber.

Figure 4 shows the RNP at a wavelength of 1.31 μm, and figure 5 - the RNP at a wavelength of 1.55 μm, measured in a prototype fiber made according to the present invention, and five other fibers made and studied with the purpose of comparison, in the process of gamma irradiation of optical fibers for 180 minutes and their relaxation for 30 minutes after the termination of irradiation.

The implementation of the invention

In figure 1, the positions indicated: 1 - core, 2 - the inner shell synthesized from a mixture of the source gaseous reagents, 3 - the outer shell formed by the material of the support pipe, 4 - a protective coating. The diameter of the outer shell d is also shown. Thus, the sheath consists of the inner sheath 2 and the outer sheath 3. The outer sheath 3 may be absent if the inner sheath 2 was not deposited on the inner surface of the support tube, or if the outer sheath 3 was removed before the optical fiber was pulled. In this case, the shell refers only to the inner shell 2.

The dashed line in Fig. 1 and Fig. 2a-c shows the boundary of two annular regions with different refractive indices in the inner shell 2. The presence of such regions is a possible embodiment of the invention, with 5 denoting the annular region of the inner shell 2 adjacent to the core 1, and position 6 is the annular region of the inner shell 2, not adjacent to the core 1, i.e. being peripheral.

An example embodiment of the invention

According to the MCVD technology, in which the core and the inner shell of the preform are synthesized by depositing quartz glass from a mixture of the initial gaseous reagents on the inner wall of the support pipe from quartz glass, six preforms were made, from which fiber optic fibers A, B, C, D, D were then pulled and E. A prototype fiber E was manufactured according to the claimed manufacturing method and is an example of the inventive radiation-resistant fiber. It is worth emphasizing that the core and the inner shell of each of the preforms were synthesized within the same technological process.

The synthesized glass was deposited in the MCVD process on a support tube made of unalloyed quartz glass F300 from Heraeus (samples A, B, D, E) or a support tube made from quartz glass doped with fluorine, F520-28 from Heraeus (samples B, D). The outer diameter of the support tubes was 25 mm and their wall thickness 2 mm.

First, on a support pipe by the known method of impregnating a porous layer of silica glass with silicon tetrafluoride (see A. N. Guryanov, M. Yu. Salgan, V. F. Khopin, A. F. Kosolapov, S. D. Semenov. “High-aperture optical fibers on based on quartz glass doped with fluorine. ”Inorganic materials, Vol. 45, No. 7, pp. 887-891 (2009)) layered layerwise an inner shell 2 of quartz glass doped with fluorine, and then the core 1 of undoped quartz glass (in the case of samples A, D, E), or quartz glass doped with fluorine (samples B, C, D).

During the deposition of each individual layer of the inner shell 2, first, when the burner moves toward the mixture of the initial gaseous reactants at a speed of 120 mm / min, a porous layer of undoped quartz glass is deposited when silicon tetrachloride and molecular oxygen are fed into the support tube (the flow rate of the latter 2500 ml / min was constant during shell synthesis for all samples). After this, the deposited porous layer was melted when the burner moved in the direction of the mixture flow of the initial gaseous reagents at the same speed and when silicon tetrafluoride (for all samples) and additional molecular oxygen (in the case of samples A and B) were fed into the support tube. As a result of this technological procedure, doping of silica glass with fluorine was achieved, so that the refractive index of the synthesized fluorosilicate inner shell 2 after the preform collapsed was approximately 0.009 lower than the refractive index of undoped silica core glass. Regions 5 and 6 with different refractive indices in the inner shell 2 were not made.

After the layers of glass of the inner shell 2 were deposited, two layers of glass of the core 1 were deposited. In this case, silicon tetrachloride and molecular oxygen were supplied to the support tube; for sample B, Freon 113 was additionally supplied at a flow rate of 2 ml / min, and for samples C and D, silicon tetrafluoride at a flow rate of 155 and 445 ml / min, respectively. The burner moved along with the flow of reagents at a speed of 120 mm / min. The deposition of silica glass with the codirectional movement of the flow of reagents and burner is called "direct deposition." As a result of the process, core 1 from quartz glass doped with fluorine in different concentrations was synthesized in samples B, C, and D, and core 1 from undoped quartz glass was synthesized in samples A, D, and E (see table).

The table shows the technological modes of obtaining a fiber preform prepared according to this invention and five other fiber preforms made for comparison, the concentration of chlorine and fluorine in the core of the preforms, measured using an electron microscope with an X-ray analyzer of chemical composition, and the diameters of the respective fibers. The technological modes indicated in the table, along with the data in the description of the example, allow reproducing the claimed objects of the invention.

The columns of the table indicate the following (from left to right): designations of blanks / optical fibers A, B, C, D, D, E, number of layers q of quartz glass doped with fluorine in shell 2, consumption of silicon tetrachloride η (SiCl ₄ ) during deposition of porous layer of shell 2, flow rate of silicon tetrafluoride η (SiF ₄ ) during penetration of the porous layer of shell 2, flow of molecular oxygen η (O ₂ ) during penetration of the porous layer of shell 2, temperature T _{1 of} penetration of the porous layer of shell 2, flow of silicon tetrachloride ξ (SiCl ₄ ) in the synthesis of core layers 1, the consumption of the molecule oxygen oxygen ξ (O ₂ ) in the synthesis of core layers 1, the consumption of silicon tetrafluoride ξ (SiF ₄ ) in the synthesis of core layers 1, the ratio of r molar flow rates of molecular oxygen and silicon tetrachloride in the synthesis of core layers 1, deposition temperature T ₂ of core layers 1, the concentration of fluorine C (F) and chlorine C (Cl) in the core 1, measured in the workpieces, and the diameter d of the optical fibers.

When the core 1 was deposited from the workpiece to the workpiece, the ratio of g molar flow rate of molecular oxygen to the molar flow rate of silicon tetrachloride was changed (see table). The commonly used ratio of 37 ... 56 was used in the case of samples A, B, D. For sample D, the oxygen consumption was intentionally underestimated (r = 20). This led to the highest chlorine content in core 1 of all samples (C (Cl) = 0.0230 wt.%). For samples B and E, the oxygen consumption was the highest (r = 70 and 75, respectively). In the case of the inventive fiber E, this led to a marked decrease in the concentration of chlorine in core 1 (C (Cl) = 0.0086 wt.%), Which turned out to be the smallest of all samples. Therefore, the inventive method of manufacturing a fiber actually allows you to suppress the entry of chlorine in the glass. In sample B, a decrease in the concentration of chlorine did not occur, since chlorine was additionally present in Freon 113, which was added to the mixture of the starting gaseous reagents. It should be noted that the addition of silicon tetrafluoride to the mixture, as expected, also suppressed the entry of chlorine into the glass of core 1 (sample D, C (Cl) = 0.0087 wt.%).

Upon completion of the deposition process of the glass core 1 tubular glass billet was subjected to compression in one or two passes of the burner at a speed of ~ 15 ... 30 mm / min, at a temperature of the outer surface of the billet more than 2200 ° C to a visually minimal diameter of the inner capillary. After that, at a temperature of ~ 2200 ° C and a burner speed of several mm / min, under the influence of surface tension forces and as a result of a temperature decrease in the viscosity of the glass, the tubular billet collapses into a solid rod, which completes the process of manufacturing the billet.

The optical fibers were pulled from the blanks with the application of a protective polymer coating 4. The drawing speed was 40 m / min and the drawing tension was 65 g.

The optical fibers were single-mode in the near-IR range with a cut-off wavelength of the first higher mode no longer than 1.65 μm. The spectrum of optical losses in the inventive fiber is shown in Fig.3. Optical losses at the most relevant wavelengths of 1.31 and 1.55 microns were 0.35 and 0.34 dB / km, which corresponds to the international standard.

Table Designation of the workpiece / fiber Shell deposition parameters 2 The deposition parameters and properties of the core 1 q η (SiCl ₄ ), ml / min H (SiF ₄ ), ml / min η (O ₂ ), ml / min T ₁ , ° C ξ (SiCl ₄ ) ml / min ξ (O ₂ ) ml / min ξ (SiF ₄ ) ml / min r T ₂ ° C C (F), wt.% C (Cl) wt.% d, μm BUT 23 400 one hundred 170 2090 45 2500 - 56 2200 0 0.0192 140 B 23 400 fifty 112 2070 fifty 3500 - 70 2300 0.20 0.0216 145 AT 17 500 fifty - 7140 67 2500 155 37 2100 0.41 0.0123 150 G 17 517 fifty - 2110 67 2500 445 37 2150 0.65 0.0087 150 D 23 400 fifty - 2070 fifty 1000 - twenty 2170 0 0,0230 133 E twenty 400 fifty - 2100 67 5000 - 75 2200 0 0.0086 125

The refractive index profile of the preforms and corresponding optical fibers is schematically shown by the solid line in Fig. 2a (for samples A, D and E), in Fig. 2b (for sample B) and in Fig. 2c (for samples C and D). The refractive index of the core 1 of the inventive radiation-resistant fiber E was 0.0002 lower than the refractive index of the outer sheath 3 formed in this particular example by the material of the support pipe F300. However, in the general case of the implementation of the inventive radiation-resistant fiber, the refractive index of the core 1 can be either greater or less than the refractive index of the outer shell 3 or equal to it, which is determined primarily by the properties of quartz glass in the support tube and the presence of alloying additives in it, such like fluoride.

The authors also found that in order to match the spot size of the mode of the inventive fiber with the size of the stains of standard fibers and to further increase the radiation resistance of the inventive fiber, it is advisable to produce an inner cladding 2 from two annular regions 5 and 6 with different refractive indices. It is advisable that the refractive index of the annular region 5 of the inner shell 2, immediately adjacent to the core 1, be lower than the refractive index of the core 1 by no more than 0.006. This annular region 5 should be synthesized by direct precipitation of quartz glass doped with fluorine, when SiCl ₄ , O ₂ and SiF _{4 are} added to the mixture of the initial gaseous reagents. Moreover, in the peripheral annular region 6 of the inner shell 2, the refractive index should be lower than the refractive index of the core 1 by at least 0.007 in order to avoid bending losses in the fiber with strong bending of the fiber. This annular region 6 should be synthesized by impregnating a porous layer of silica glass with silicon tetrafluoride.

Optical fibers A, B, C, G, D, E were irradiated from a cobalt-60 gamma radiation source for 180 minutes at a dose rate of 0.75 Gy / s and at room temperature to a dose of 8.1 kGy, and the value was measured during irradiation RNP. After 180 minutes, irradiation was stopped, and RNP was measured for another 30 minutes in the absence of irradiation.

As can be seen in figure 4, the suppression of the entry of chlorine into the glass in the claimed invention leads to the suppression of RNP-1: in contrast to the fiber D with a high chlorine content and therefore a pronounced RNP-1 (monotonous increase in RNP with increasing dose), and the claimed optical fiber E RNP-1 was not observed.

The inventive methods allow to suppress RNP-2. This is proved by comparing in FIGS. 4 and 5 the RNP curves for a fiber G with a fluorine-doped silica glass core 1 and the RNP curves for the inventive light guide E with an undoped quartz glass core 1. These fibers contain almost the same small amount of chlorine, however, the RNP is noticeably greater for fiber G than for the inventive fiber E (Fig. 3). In this case, according to the form of RNP in sample G, we can conclude that it is due to RNP-2, a distinctive feature of which is a nonmonotonic dose-dependent relationship. Meanwhile, in the claimed fiber E, the signs of a nonmonotonic course of the RNP practically do not appear depending on the dose, therefore RNP-2 is almost completely suppressed in the claimed fiber.

The claimed fiber is suppressed and RNP-3. To verify this, we made the following observation: 15 minutes after the completion of the above irradiation, the RNP was compared in fibers A, B, C, D, D, E at a wavelength of 1.7 μm, at which RNP-3 is the determining mechanism. It turned out that for the claimed fiber E, as well as for the fibers B, C, D with core 1 of quartz glass doped with fluorine, the RNP was about 3 dB / km, for fiber A - 6 dB / km, for fiber D - 10 dB / km. Therefore, the claimed method allows to suppress RPN-3 no worse than the method consisting in doping the core 1 with fluorine.

Thus, the claimed fiber suppressed all three mechanisms of RNP (RNP-1, 2 and 3). As follows from figure 4 and 5, at both of the most used optical communication wavelengths of 1.31 and 1.55 μm, the inventive optical fiber E shows the smallest RNP of the six studied optical fibers A, B, C, G, D, E, t. e. highest radiation resistance.

In an example of the implementation of a close analogue to US Pat. The inventive light guide E showed at the same radiation dose almost the same RNP (10 dB / km, FIG. 4), but, importantly, when the dose rate was 3.4 times higher (0.75 Gy / s). It is known that in optical fibers with an undoped core and a core doped with fluorine, the RNP greatly decreases with decreasing dose rate. Therefore, with a dose rate of 0.22 Gy / s in the inventive fiber, the RNP would be significantly less than with a dose rate of 0.75 Gy / s and, therefore, less than that of the indicated analogue. Thus, the inventive fiber is superior in radiation resistance to the optical fiber-analogue.

Attention should be paid to the RNP in fiber B, also obtained with a significant excess of oxygen in the mixture of the starting reagents (r = 70), which in this case also contained freon 113. As a result, the core was doped with fluorine. However, it is easy to see that the relatively high radiation resistance of fiber B is not due to an admixture of fluorine in the core (fibers B and D contained even more fluorine, but showed less radiation resistance, see Figs. 4, 5 and the table), but a significant excess of oxygen in the synthesis of the core. Therefore, in addition to a method of manufacturing a radiation-resistant fiber with a core of undoped silica glass, variants of a method for increasing the radiation resistance of a fiber fiber manufactured with a core of undoped or fluorine-doped silica glass are also claimed.

The inventive radiation-resistant troubled fiber, the method of its manufacture and variants of the method of increasing the radiation resistance of a fiber can be implemented using other known technological processes for the chemical deposition of silica glass from a mixture of initial gaseous reagents, such as FCVD, VAD, OVD, PCVD, SPCVD. It should be noted that the FCVD process, also consisting in the chemical deposition of silica glass on the inner surface of the support pipe, is essentially just an embodiment of the MCVD process, differing only in the way the support pipe is heated (see A.A. Malinin, AS Zlenko, LG Akhmetshin, SL Semjonov "Furnace chemical vapor deposition (FCVD) method for special optical fibers fabrication", Proc. SPIE, vol. 7934, paper 793418 (2011)).

The radiation resistance of the inventive fiber light guide meets modern requirements for radiation-resistant fiber light guides in their practical applications. For example, the inventive radiation-resistant fiber optic fiber can be used in the Large Hadron Collider, where the fiber is exposed to nuclear and ionizing radiation. It is known that this application requires an optical fiber with optical losses at an irradiation dose of 100 kGy of not more than 7 dB / km at a low dose rate (less than 0,0003 Gy / s) at a working wavelength of 1.31 μm (see T. Wijnands , LK De Longe, J. Kuhnhenn, SK Hoeffgen, U. Weinand "Optical absorption in commercial single mode fibers in a high energy physics radiation field", IEEE Transactions on Nuclear Science, vol. 55, pp. 2216-2222 (2008) ) The authors conducted an additional experiment in which the inventive fiber E was irradiated to a dose even greater - 1.31 MGy (dose rate was 0.73 Gy / s). Optical losses in the inventive radiation-resistant fiber optic fiber E 6 days after such exposure were only 10 dB / km. Given that the irradiation was carried out to a dose more than an order of magnitude greater than the dose when used in the Large Hadron Collider, and at a much higher dose rate, we can conclude that in the above operating conditions the claimed radiation-resistant fiber optic fiber would show many times smaller RNP. Thus, it satisfies the requirements for a fiber waveguide intended for practical use in the Large Hadron Collider.

Claims (32)

1. A radiation-resistant fiber optic fiber containing a core and a sheath based on quartz glass and a protective coating, the core is made of undoped quartz glass, the cut-off wavelength of the first highest mode of the specified fiber does not exceed 1.7 μm, and the chlorine content in its core does not exceed 0.01 weight percent. 2. The fiber according to claim 1, in which the chlorine content in the core exceeds 1 · 10 ^-5 weight percent. 3. The fiber according to claim 1, in which the amplitude of the absorption band of OH groups at a wavelength of 1.38 μm in the spectrum of the optical loss of the fiber does not exceed 9 dB / km. 4. The fiber according to claim 1, in which the sheath consists of an outer and inner sheath or only of the inner sheath. 5. The optical fiber according to claim 4, in which the inner shell is made of quartz glass doped with fluorine. 6. The fiber according to claim 5, in which the inner shell contains an annular region adjacent to the core, the refractive index of which is less than the refractive index of the core by no more than 0.006. 7. The fiber according to claim 6, in which the sheath contains an annular region, the refractive index of which is less than the refractive index of the core by more than 0.007. 8. The fiber according to claim 1, which is microstructured. 9. The optical fiber according to claim 1, which is photonic crystal. 10. The fiber according to claim 1, which is birefringent. 11. A method of manufacturing a radiation-resistant silica glass fiber based on a silica glass, the method comprising manufacturing a preform containing a core and a sheath using a single process of chemical deposition of silica glass from a mixture of the starting gaseous reagents and subsequent drawing from the fiber preform, the core glass being synthesized from the mixture silicon tetrachloride and molecular oxygen and provide an excess of the molar flow rate of molecular oxygen over the molar flow rate of silicon tetrachloride necks least 75 times. 12. The method according to claim 11, in which the chemical deposition of silica glass is carried out on the inner surface of the support pipe from undoped or fluorine-doped silica glass, which is the outer sheath of the fiber. 13. The method according to item 12, in which the outer shell of the preform is removed before drawing the fiber. 14. The method according to claim 11, in which the shell is synthesized from a mixture of source gaseous reagents containing silicon tetrachloride, silicon tetrafluoride and molecular oxygen. 15. The method according to 14, in which the region of the shell adjacent to the core is synthesized by direct deposition of quartz glass doped with fluorine, and the peripheral region of the shell is synthesized by impregnating a porous layer of silica glass with silicon tetrafluoride. 16. A method of increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is prepared by ohmic deposition of silica glass from a mixture of initial gaseous reagents containing silicon tetrachloride and molecular oxygen, and in this mixture the molar excess the flow rate of molecular oxygen over the molar flow rate of silicon tetrachloride is at least 75 times. 17. The method according to clause 16, in which the shell of the preform is synthesized by chemical deposition of silica glass from a mixture of the source gaseous reagents. 18. The method according to 17, in which the synthesis is carried out from a mixture of gaseous reactants containing silicon tetrachloride, silicon tetrafluoride and molecular oxygen. 19. The method according to p, in which the region of the shell adjacent to the core is synthesized by direct deposition of quartz glass doped with fluorine, and the peripheral region of the shell is synthesized by impregnating a porous layer of silica glass with silicon tetrafluoride. 20. The method according to 17, in which the chemical deposition of silica glass during the synthesis of the core and the shell is conducted on the inner surface of the support pipe from undoped or fluorine-doped silica glass. 21. A method of increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is prepared by chemical deposition of silica glass from a mixture of the initial gaseous reagents containing silicon tetrachloride, molecular oxygen and silicon tetrafluoride, moreover, in said mixture provide an excess of the molar flow rate of molecular oxygen over the molar flow rate of silicon tetrachloride at least 70 times. 22. The method according to item 21, in which the shell of the preform is synthesized by chemical deposition of silica glass from a mixture of the source gaseous reagents. 23. The method according to item 22, in which the synthesis is carried out from a mixture of gaseous reactants containing silicon tetrachloride, silicon tetrafluoride and molecular oxygen. 24. The method according to item 23, in which the region of the shell adjacent to the core is synthesized by direct deposition of quartz glass doped with fluorine, and the peripheral region of the shell is synthesized by impregnating a porous layer of silica glass with silicon tetrafluoride. 25. The method according to item 22, in which the chemical deposition of quartz glass during the synthesis of the core and the shell lead to the inner surface of the support pipe from undoped or fluorine-doped silica glass. 26. The method according to item 21, in which freon 113 is added to the mixture of source gaseous reactants. 27. A method of increasing the radiation resistance of a silica glass fiber containing a core, a sheath and a protective coating, the core of which is prepared by chemical deposition of silica glass from a mixture of the initial gaseous reagents containing silicon tetrachloride, molecular oxygen and freon 113, and moreover, in said mixtures provide an excess of the molar flow rate of molecular oxygen over the molar flow rate of silicon tetrachloride at least 70 times. 28. The method according to item 27, in which the shell of the preform is synthesized by chemical deposition of silica glass from a mixture of the source gaseous reagents. 29. The method according to p, in which the synthesis is carried out from a mixture of gaseous reagents containing silicon tetrachloride, silicon tetrafluoride and molecular oxygen. 30. The method according to clause 29, in which the region of the shell adjacent to the core is synthesized by direct deposition of quartz glass doped with fluorine, and the peripheral region of the shell is synthesized by impregnating a porous layer of silica glass with silicon tetrafluoride. 31. The method according to p, in which the chemical deposition of quartz glass during the synthesis of the core and the shell is conducted on the inner surface of the support pipe from undoped or fluorine-doped silica glass. 32. The method according to item 27, in which silicon tetrafluoride is added to the mixture of source gaseous reactants.

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