Glass body for optical fiber, use thereof, optical fiber, and method of making it

Abstract: A glass body for optical fiber containing GeO 2 -SiO 2 glass in a core portion thereof, in which the GeO 2 -SiO 2 glass has an absorbance at 5.16 eV of at least 1/mm but not higher than 2.5/mm or in which concentration of Ge 2+ contained in the GeO 2 -SiO 2 glass substantially lies within the range of 1.1 × 10 -9 to 2.8 × 10 -9 mol/mm 3 as calculated by the following general equation: A = ε 5.16ev · C (Ge2+) · l wherein A is absorbance which is expressed by A = -log T (T being transmittance) and normalized per 1 mm of optical path length, C (Ge2+) is Ge 2+ concentration, ε 5.16ev is absorption coefficient (l/mol/cm), and l is optical path length. This invention is also directed to the use of such a glass body, an optical fiber and a method of preparing it.

Description: The present invention relates to a glass body for optical fiber, the use thereof, a method of making the optical fiber, and an optical fiber obtainable by such a method. In The Transactions of the Institute of Electronics, Information and Communication Engineers, C-I, vol. J72-C-I, No. 1, pp. 45-52 (1989), it is described that, when absorption at a wavelength of 0.63 µm (non-bridging oxygen hole center, referred to as "NBOHC" hereinafter, i.e., ≡Si-O · ) exists in a silica glass type optical fiber comprising GeO 2 -SiO 2 glass as its core, hydrogen diffuses into the optical fiber, thereby increasing absorption loss at 1.38 µm (Si-OH) and 1.53 µm (attributable to several sources according to different theories). This publication discloses that the absorption at 0.63 µm (Si-O · ) is generated due to the fact that alkali elements in the glass diffuse into the fiber at a drawing step, thereby forming a bond (≡Si-O-Na + ) which becomes a source of defect absorption, and that the absorption is formed as a glass network is severed upon high tension and high drawing rate at the time of drawing. Such an optical fiber with a large absorption at a wavelength band of 0.63 µm has been disadvantageous in that, when hydrogen diffuses into the glass, it yields a large increase in absorption at 1.38 µm and 1.53 µm, thereby adversely affecting transmission loss at communication wavelength bands of 1.3 µm and 1.55 µm. Currently, optical fibers are manufactured such that impurities such as alkali and transition metal are decreased as much as possible while their drawing condition is selected so as to suppress the absorption at 0.63 µm as much as possible. Nevertheless, due to the instability in glass structure caused by addition of GeO 2 thereto, the absorption at 0.63 µm of the optical fiber comprising GeO 2 -SiO 2 glass as its core cannot have been sufficiently suppressed as compared with an optical fiber whose core is made of pure silica glass. H. Hosono et al describe in Appl. Phys. Lett. 63 (4), 26 July 1993, pp. 479-481 that the heat treatment of a 5 GeO 2 -95SiO 2 glass preform in a H 2 atmosphere at 500°C for 70 h increased intensities of an absorption band centered at 5 eV by a factor of approx. 3. A.N. Gur'yanov et al describe in Fizika i Khimiya Stekla, Vol. 12, No. 3, pp. 359-364, May - June 1986, that they have measured the radial distributions of Ge and Cl and the UV absorption and intensity of luminescence in the 390 nm band in fiber light-guides (FL) blanks. They found that the absorption bands at ∼ 242 and 330 nm and the luminescence band at ∼ 390 nm belong to the same center. T.E. Tsai et al report in Appl. Phys. Lett. 64 (12), 21.03.1994, pp. 1418-1483 about the study of the 5.16 keV absorption band observed in silica and Ge-doped silica using optical and electron spin resonance measurements. The band was observed only in samples containing Ge, suggesting that it is related to the Ge impurity in silica. K. Susa et al describe in Journal of Non-Crystalline Solids 128 (1991) 118-125 that Ge-doped silica fibers exhibit a typical transmission loss of 30-20 dB/km in a wavelength range of 0,63 to 0,83 µm. EP-A-0 495 605 reveals a Ge-silicate optical fiber having a loss at a wavelength of 330 nm of at least 30 db/m·mole% GeO 2 . EP-A-0 171 537 discloses a method for producing a glass preform having less structural defects due to Ge 2+ than the conventionally produced ones. In an optical fiber fabricated from this glass preform absorption at by Ge 2+ did not increase attenuation of light transmission at a wavelength of 0,85 µm. EP-A-0 164 127 is directed to a method for producing a glass preform for use in the fabrication of an optical fiber, wherein a deficiency due to the presence of Ge 2+ is substantially decreased. The relationship between Ge 2+ and the increase of attenuation of light transmission (Δα dB/km) at a wavelength of 1,41 and 1,55 µm is discussed. The object of the present invention is to provide a glass body and optical fiber which can sufficiently suppress absorption at 0,63 µm. It is also an object to provide a method for preparing an optical fiber. The present invention provides a glass body for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, wherein the fluorine-containing GeO 2 -SiO 2 glass has an absorbance at 5.16 eV of at least 1/mm but not higher than 2.5/mm. The present invention also provides the use of such a glass body as a matrix for forming the optical fiber by melt-drawing. Also, the present invention provides a glass body for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, wherein concentration of Ge 2+ contained in the fluorine-containing GeO 2 -SiO 2 glass substantially lies within the range of 1.1 × 10 -9 to 2.8 × 10 -9 mol/mm 3 as calculated by the following general equation: A = ε 5.16ev · C (Ge2+) · l wherein A is absorbance which is expressed by A = -log T (T being transmittance) and normalized per 1 mm of optical path length, C (Ge2+) is Ge 2+ concentration, ε 5.16ev is absorption coefficient (l/mol/cm), and l is optical path length. Further, the present invention provides a method of selecting a matrix for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, which method comprises the steps of selecting, before manufacturing an optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, a glass body in which the fluorine-containing GeO 2 -SiO 2 glass has an absorbance at 5.16 eV of at least 1/mm but not higher than 2.5/mm or in which concentration of Ge 2+ contained in the glass substantially lies within the range of 1.1 × 10 -9 to 2.8 × 10 -9 mol/mm 3 as calculated by the following general equation: A = ε 5.16ev · C (Ge2+) · l wherein A is absorbance which is expressed by A = -log T (T being transmittance) and normalized per 1 mm of optical path length, C (Ge2+) is Ge 2+ concentration, ε 5.16ev is absorption coefficient (l/mol/cm), and l is optical path length; and using thus selected glass body as a matrix for forming the optical fiber by melt-drawning. Also, the present invention provides a method of making an optical fiber comprising the steps of melting a matrix for optical fiber comprising the foregoing glass body for optical fiber in accordance with the present invention in an inert gas atmosphere, e.g. nitrogen gas, under a condition where the temperature within the furnace is about 2000°C to 2200°C, preferably about 2100°C ; and then melt-drawing thus melt glass body at an drawing rate of 100 to 2,000 m/min. so as to yield an optical fiber. Here, "glass body for optical fiber" in the present invention encompasses not only the matrix for optical fiber used in the drawing step but also so-called intermediates which are forms in the manufacturing step for making the matrix. The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings. Fig. 1 is a chart showing an absorption peak at 5.16 eV (wavelength of 0.240 µm); Fig. 2 is a chart showing a relationship between absorption at 5.16 eV and Δ absorption at 0.63 µm; Fig. 3 is a chart showing a relationship between Δ absorption at 0.63 µm and increase in loss at 1.38 µm after hydrogen processing; Fig. 4 is a chart showing a relationship between Δ absorption at 0.63 µm and increase in loss at 1.53 µm after hydrogen processing; Fig. 5 is a chart showing temperature dependency of equilibrium constant in each chemical reaction; Fig. 6 is a model chart for explaining causes of absorption at 5.16 eV; and Figs. 7A and 7B are schematic views showing an apparatus for measuring Δ absorption at 0.63 µm. Fig. 1 is a graph showing an absorption peak (with a half-width of 0.5 × ΔA (eV)) at 5.16 eV (wavelength of 0.240 µm) which the inventors have taken account of. Here, absorbance A is expressed by A = -log T (T being transmittance) and normalized per 1 mm of optical path length. Also, difference ΔA from a baseline on the graph is defined as absorbance and assumed to be within the range of at least 1/mm but not higher than 2.5/mm. The inventors have attributed the peak at 5.16 eV to Ge 2+ since the half-width of the peak substantially coincides with the center wavelength of Ge 2+ disclosed in Table II of H. Hosono et al., Physical Review , B, vol. 46, No. 18 (1992), pp. 11445-11451. This publication is incorporated herein by reference. Absorption at 5.16 eV in a GeO 2 -SiO 2 glass body was measured, an optical fiber comprising this glass body as its core was prepared, Δ absorption at 0.63 µm of this optical fiber was measured, both of these absorption data were plotted on a graph, and their correlation was investigated. Fig. 2 shows the result thereof. As in the case of absorption at 5.16 eV, Δ absorption at 0.63 µm is analyzed as a difference from the baseline. Figs. 3 and 4 show relationships between Δ absorption at 0.63 µm before hydrogen processing and increases in absorption loss at 1.38 µm and 1.53 µm after the hydrogen processing, respectively. Here, the hydrogen processing of optical fibers was effected for one week at room temperature in an atmosphere of 1% H 2 . For the fluorine-containing GeO 2 -SiO 2 glass bodies, the absorption at 5.16 eV thereof was measured as follows. Namely, the glass body was cut into a thickness of 1 mm, its cutting plane was polished into a mirror surface, and absorption spectrum in the core portion of the resulting sample was measured by a normal ultraviolet/visible spectrophotometer. After an optical fiber comprising fluorine-containing GeO 2 -SiO 2 glass body as its core was prepared, its Δ absorption at 0.63 µm was measured as explained in the following. As shown in Figs. 7A and 7B, within a range defined by V-shaped grooves 2, transmittance of light from a light source 1 through an optical fiber 5 is measured by a pass meter 6, and then Δ absorption at 0.63 µm is computed from the following equation: (P out - P in )/sample length [dB/km] wherein P out is transmittance of light through the sample 5 to be measured when its length L out = 300 m, whereas P in is transmittance of light through the sample 5 when its length L in = 1 m. The range of light measured is 500 to 800 nm, with a measurement pitch of 10 nm. As the light source 1, a xenon (Xe) lamp was used. From the results shown in Figs. 3 and 4, the inventors have clarified that Δ absorption at 0.63 µm has a threshold near 7 dB/km and that, at this threshold or higher value, the absorption at 1.38 µm increases while absorption loss at 1.53 µm is generated. Further, the inventors have found out from Fig. 2 that, in order to suppress Δ absorption at 0.63 µm to 7 dB/km or less, it is-appropriate for the absorbance at 5.16 eV to be set at 1 to 2.5/mm. With respect to fluorine-containing GeO 2 -SiO 2 glass, the inventors have found out that, when glass (F-GeO 2 -SiO 2 ) whose absorbance at 5.16 eV is 1 to 2.5/mm is used to prepare a matrix for optical fiber and then drawn to form an optical fiber, absorption losses at 1.38 µm and at 1.53 µm can be suppressed, whereby an optical fiber which is excellent in hydrogen resistance can be obtained. Mechanisms by which absorption at 0.63 µm is generated and thus generated absorption at 0.63 µm (≡Si-O · ) becomes absorptions at 1.38 µm and 1.53 µm upon hydrogen processing are considered by the inventors as follows. Fig. 5 shows temperature dependencies of equilibrium constants in the following three decomposition reactions (1) to (3): (1) GeO 2 → GeO + 1/2 O 2 (2) 1/2 GeO 2 + 1/2 SiO 2 → 1/2 GeO + 1/2 SiO + 1/2 O 2 (3) SiO 2 → SiO + 1/2 O 2 From Fig. 5, it is presumed that decomposition reaction for GeO 2 is most likely to occur and that, in a low-concentration reaction such as that of glass defects, the decomposition reaction occurs at a temperature of about 1,000°C. Change of GeO 2 into GeO is equivalent to reduction of Ge 4+ into Ge 2+ . Absorption at 5.16 eV is considered to capture thus reduced Ge 2+ (GeO). When studied from the viewpoint of glass structure, this phenomenon is represented by a model shown in Fig. 6. Upon the reaction shown in Fig. 6, ≡Si-O · is generated together with Ge 2+ . Also, it is considered that oxygen is released, and thus released free oxygen chemically combines with other kinds of defects (≡Si · , i.e., E' center, and ≡Ge-Si≡ or ≡Si-Si≡, i.e., oxygen-vacancy, which generates E' center when released) present in the glass, thereby newly generating ≡Si-O · . Accordingly, it is presumed that, even in the case where Ge concentration is constant, excess oxygen becomes greater as the number of Ge 2+ defects is larger. When F (fluorine) is added to GeO 2 -SiO 2 glass, ≡Si · (E' center) or oxygen-vacancy such as ≡Ge-Si≡ or ≡Si-Si≡ reacts with F to form Si-F, thereby effectively suppressing the generation of ≡Si-O · by free oxygen. Further, when hydrogen diffuses into glass, the generated NBOHC reacts with hydrogen to form Si-OH (see the following equation). ≡Si-O · + 1/2 H 2 → ≡Si-OH (absorption at 1.38 µm) Also, when the binding energy of Si-O and that of Ge-O are compared with each other, the former is 400 KJ/mol whereas the latter is 330 KJ/mol. Accordingly, Si-O exists more stably. At the drawing step where temperature becomes higher than that in the heat treatment step for fine particles of glass where the fine particles are subject to a temperature of 1,000 to 1,600°C, the glass matrix is subjected to a high temperature of 2,000 to 2,200°C. Accordingly, under a drawing condition which yields high tension and high drawing rate, Ge-O bond in ≡Ge-O-Si≡ linkage is cut off. Since Ge 2+ in Ge is stable, as the bond is cut off, glass defects of =Ge 2+ , ≡Si-O · , and are consequently generated. As mentioned above, ≡Si-O · is a defect of NBOHC and reacts with hydrogen to generate Si-OH, thereby increasing absorption loss at 1.38 µm. On the other hand, the defect of seems to react with hydrogen to form an intramolecular hydrogen bond, thereby generating absorption at 1.53 µm. This intramolecular hydrogen bond has an absorption at 3,400 to 3,200 cm -1 which is attributable to a harmonic of 1.53 µm. Here, when the relationship between absorbance A and Ge 2+ concentration is determined with reference to the disclosure in a publication (H. Hosono et al., "Nature and Origin of the 5-eV Band in SiO 2 :GeO 2 Glasses," Physical Review , vol. 46, No. 118, pp. 11445-11451), the following equation is obtained: A = ε 5.16ev · C (Ge2+) · l wherein ε 5.16ev is absorption coefficient (l/mol/cm), C (Ge2+) is Ge 2+ concentration, and l is optical path length. Assuming that ε 5.16ev = 9 × 10 3 (using the above-mentioned publication, p. 11449, left column, lines 6 to 7) and l = 1 mm (measurement condition), Ge 2+ is determined as Ge 2+ = A/ε 5.16ev /l. As a result of calculations using the values listed above, the Ge 2+ concentration corresponding to absorbance A = 1 to 2.5/mm -1 becomes 1.1 × 10 -9 to 2.8 × 10 -9 mol/mm 3 . The above-mentioned publication is incorporated herein by reference. As explained in the foregoing, the inventors have clarified that the absorption defect (≡Si-O · ) at 0.63 µm is closely related to the absorber at 5.16 eV. When glass manufacturing conditions (deposition condition for fine particles of glass, sintering condition, etc) are optimized so as to control the absorbance at 5.16 eV as defined in the claimed glass bodies for optical fiber and optical fibers, generation of glass defects can be decreased so as to sufficiently suppress the absorption at 0.63 µm. Thus, in accordance with the present invention, a glass body for manufacturing an optical fiber which is excellent in hydrogen resistance and has less glass defects can be selected. Also, without evaluation of absorption loss on the shorter wavelength side as an optical fiber, its hydrogen characteristic can be estimated in the glass state. Accordingly, absorption loss can be evaluated easily and efficiently. The concentration of Ge 2+ contained in GeO 2 -SiO 2 glass is substantially calculated by the above-mentioned general equation and lies within the range of 1.1 × 10 -9 to 2.8 × 10 -9 mol/mm 3 . (Reference) Example 1 GeO 2 -SiO 2 glass whose absorbance at 5.16 eV was 1.7/mm, as a matrix for optical fiber, was melt in a melting furnace in an inert gas atmosphere under a condition where the temperature within the furnace was about 2,000°C, and then thus melt glass body was melt-drawn at a drawing rate of about 200 m/min., whereby an optical fiber having a core diameter of about 8.5µm was obtained. (Reference) Example 2 GeO 2 -Sio 2 glass in which concentration of Ge 2+ contained therein was 1.89 × 10 -9 mol/mm 3 ., as a matrix for optical fiber, was melt in a melting furnace in an inert gas atmosphere under a condition where the temperature within the furnace was about 2,100°C, and then thus melt glass body was melt-drawn at a drawing rate of about 500 m/min., whereby an optical fiber having a core diameter of about 8.5µm was obtained. Here, the concentration of Ge 2+ contained in the GeO 2 -SiO 2 glass was computed according to the following general equation: A = ε 5.16ev · C (Ge2+) · l wherein A is absorbance which is expressed by A = -log T (T being transmittance) and normalized per 1 mm of optical path length, C (Ge2+) is Ge 2+ concentration, ε 5.16ev is absorption coefficient (l/mol/cm), and l is optical path length. Example 3 Fluorine-containing GeO 2 -SiO 2 glass whose absorbance at 5.16 eV was 1.5/mm was used as a matrix for optical fiber, and under a condition identical to that of Example 1, an optical fiber having a core diameter of about 8.5µm was obtained. Example 4 Fluorine-containing GeO 2 -SiO 2 glass in which concentration of Ge 2+ contained therein was 1.67 × 10 -9 mol/mm 3 was used as a matrix for optical fiber, and under a condition identical to that of Example 2, an optical fiber having a core diameter of 8.5µm was obtained. Here, the concentration of Ge 2+ contained in the fluorine-containing GeO 2 -SiO 2 glass was computed according to the general equation used in Example 2. As shown in Fig. 2, it was confirmed that increase in peaks of transmission loss at an absorption band of 0.63 µm was suppressed in the optical fibers obtained in Examples 1 to 4. Comparative Example 1 GeO 2 -SiO 2 glass whose absorbance at 5.16 eV was 0.5/mm was used as a matrix for optical fiber, and under a condition identical to that of Example 1, an optical fiber having a core diameter of 8.5µm was obtained. Comparative Example 2 GeO 2 -SiO 2 glass in which concentration of Ge 2+ contained therein was 0.56 × 10 -9 mol/mm 3 was used as a matrix for optical fiber, and under a condition identical to that of Example 2, an optical fiber having a core diameter of 8.5µm was obtained. Here, the concentration of Ge 2+ contained in the GeO 2 -SiO 2 glass was computed according to the general equation used in Example 2. Comparative Example 3 Fluorine-containing GeO 2 -SiO 2 glass whose absorbance at 5.16 eV was 2.8/mm was used as a matrix for optical fiber, and under a condition identical to that of Example 1, an optical fiber having a core diameter of 8.5µm was obtained. Comparative Example 4 Fluorine-containing GeO 2 -SiO 2 glass in which concentration of Ge 2+ contained therein was 3.11 × 10 -9 mol/mm 3 was used as a matrix for optical fiber, and under a condition identical to that of Example 2, an optical fiber having a core diameter of 8.5µm was obtained. Here, the concentration of Ge 2+ contained in the fluorine-containing GeO 2 -SiO 2 glass was computed according to the general equation used in Example 2. As shown in Fig. 2, increase in peaks of transmission loss at an absorption band of 0.63 µm was not suppressed in the optical fibers obtained in Comparative Examples 1 to 4.

Claims: A glass body for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, said fluorine-containing GeO 2 -SiO 2 glass having an absorbance at 5.16 eV of at least 1/mm but not higher than 2.5/mm. A glass body for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof, said fluorine-containing GeO 2 -SiO 2 glass containing Ge 2+ with a concentration which substantially lies within a range of 1.1 x 10 -9 to 2.8 x 10 -9 mol/mm 3 . A glass body according to claim 2, said fluorine-containing GeO 2 -SiO 2 glass containing Ge 2+ with a concentration which substantially lies within a range of 1.1 x 10 -9 to 2.8 x 10 -9 mol/mm 3 as calculated by the following general equation: A = ε 5.16eV · C (Ge2+) · 1 wherein A is absorbance which is expressed by A = -logT (T being transmittance) and normalized per 1 mm of optical path length, C (Ge2+) is Ge 2+ concentration, ε 5.16eV is absorption coefficient (1/mol/cm), and 1 is optical path length. Use of a glass body as claimed in any one of the claims 1 to 3, as a matrix for forming the optical fiber by melt-drawing. A method of making an optical fiber comprising the steps of: selecting a matrix for optical fiber containing fluorine-containing GeO 2 -SiO 2 glass in a core portion thereof as defined in any one of the claims 1 to 3; melting said matrix for optical fiber in an inert gas atmosphere under a condition where the temperature within the furnace is about 2,000 to 2,200°C; and then melt-drawing said melt glass body at a drawing rate of 100 to 2,000 m/min. so as to yield an optical fiber. An optical fiber obtainable by a process according to claim 5.

Inventors: ENOMOTO TADASHI, ISHIKAWA SHINJI, OHGA YUICHI, ENOMOTO, TADASHI, ISHIKAWA, SHINJI, OHGA, YUICHI, Enomoto, Tadashi, c/o Sumitomo Electric Ind., Ltd., Ishikawa, Shinji, c/o Sumitomo Electric Ind., Ltd., Ohga, Yuichi, c/o Sumitomo Electric Ind., Ltd.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1996
Main classification: C03C13
IPC codes: G02B6, C03C13, C03B37, C03C3