JP2767439B2 - Optical fiber manufacturing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of drawing a quartz optical fiber preform containing a relatively large amount of an additive such as germanium or phosphorus in a core with low loss. is there.

(Prior art) When an optical fiber such as a dispersion-shifted fiber for 1.55 μm transmission containing a relatively large amount of additives in a core is obtained by drawing from a base material, conventionally, a method of drawing with a relatively high drawing tension is used. It was described that transmission loss could be reduced.

(Problems to be Solved by the Invention) However, regarding the magnitude of the drawing tension, there is no clear guide as to how much drawing tension should be drawn. As a result, the mechanical strength of the optical fiber after drawing is reduced, and the optical fiber may be broken by applying a slight tensile stress. Such a relationship between the drawing tension and the mechanical strength is influenced by the heating conditions of the base material at the time of drawing and the surrounding atmosphere and is not necessarily determined uniquely, but is obtained by fixing the drawing conditions. The results obtained are shown in FIG. 1 as shown in FIG. It is a result of a tensile test of 100 quartz-based dispersion-shifted fibers having an outer diameter of 125 μm and a length of 20 m. Regarding the loss, as a result of drawing various 1.55 μm dispersion-shifted fiber preforms, histograms as shown in FIGS. 2 and 3 were obtained. FIG. 2 shows a loss frequency distribution when a fiber is formed by fixing the drawing tension to a constant 35 g, and FIG. 3 shows a loss frequency when the fiber is formed by changing the drawing tension to 60 to 150 g. The distribution is shown. Thus, 1.55 μm transmission originally indicates a wavelength at which the theoretical loss of a silica-based glass fiber is low, and its value is reduced by half at a high transmission loss. The results in FIGS. 2 and 3 are far from satisfactory given this point.

(Means for Solving the Problems) In view of the above, the present invention provides a quartz-based glass whose refractive index distribution is controlled by adding a large amount of an additive such as germanium or phosphorus which increases the thermal expansion coefficient of quartz glass. An optical fiber preform having a cladding member made of quartz glass provided around a core member made of glass so as to change the refractive index of quartz glass by about 0.2% at most even with an additive is represented by the following formula (1). A method for producing an optical fiber, characterized in that the optical fiber is drawn with a drawing tension or higher.

t ≧ (β core−β clad) × E × S Formula (1) t: Drawing tension β core: Thermal expansion of core glass from normal temperature to slow cooling temperature β cladding: Cladding from normal temperature to slow cooling temperature of core glass Thermal expansion of glass E: Young's modulus of clad glass S: Cross-sectional area of clad glass The large amount of dopant added to the core member in the present invention means that the refractive index of quartz glass is at least 0.5%.
It means that the amount is increased to an extent, for example, 7.5 wt% or more when only GeO ₂ is added.

The clad member may be pure quartz glass or GeO ₂ ,
Even if dopants such as P ₂ O ₅ , F, and CI are added, the refractive index is changed by about 0.2% from that of quartz glass, and the difference in thermal expansion coefficient between the core center member and the clad member is at least small. It is intended for objects that differ greatly by about 2 × 10 ^-4 .

(Operation) With the above configuration, the difference in thermal expansion between the core and the clad can be offset by the tensile strain generated during the drawing of the clad glass, thereby achieving an extremely low loss of the obtained optical fiber. Can be.

In addition, as a result of investigating various dispersion shift fibers, it was found that the required drawing tension was determined by the addition amount of germanium in the portion having the highest thermal expansion. Further, it has been found that in this kind of fiber using germanium as a main dopant, the loss does not change at all beyond the value represented by the equation (1). The relationship between the drawing tension and the transmission loss can not always be explained sufficiently physically and scientifically, but is currently estimated as follows. That is, in a glass to which a dopant such as germanium is added, fluctuation of the refractive index is inherent in the structure, and this is what is called Rayleigh scattering depending on the relationship between the magnitude of the fluctuation and the wavelength of light. However, when a tensile stress is applied to such a glass, the fluctuation of the refractive index further increases because the photoelastic constant is locally different. However, in the compression direction, this fluctuation acts rather in a suppressed direction, so that no loss increases at this time. Therefore, the transmission loss of the fiber can be kept stable with the drawing tension of the equation (1) or more.

When the base material is drawn based on the formula (1), the remaining problem is a decrease in the mechanical strength of the fiber glass due to the high tension drawing, but in order to prevent this, the drawing atmosphere must be cleaned. It is sufficient to take ordinary measures such as reducing the dust level in the vicinity of the glass surface, and reducing the humidity of the atmosphere to prevent the growth of scratches generated on the glass surface.

Furthermore, since the above equation (1) does not depend on the drawing speed, it is 800 m
Even recent speedups such as / minute can be handled without any problems.

Example In order to obtain a low-loss dispersion-shifted fiber having a refractive index distribution shown in FIG. 4, a drawing tension was obtained based on the equation (1). In FIG. 4, reference numeral 1 denotes a first core having a refractive index distribution which changes sharply in the radial direction, and has a diameter of about 4 μm.
m and 2 are second cores located around the first core and having an outer diameter of about 14 μm, and have a constant refractive index equal to the refractive index of the outermost peripheral portion of the first core. Reference numeral 3 denotes a cladding having an outer diameter of 125 μm which is located around the second core and has a refractive index slightly lower than that of the second core. Specifically, the refractive index difference の between the first core 1 and the clad 3 is 1.0%, and the refractive index difference between the second core 2 and the clad 3 is 0.13%.
It is given by adding Ge. Now, regarding the β _core , as described above, the core is composed of the first and the second and has a distribution of the refractive index. However, when it is considered that △ = 1.0%, the corresponding SiO ₂ —GeO The GeO ₂ concentration of the _two- diameter glass is 15 wt% from FIG. Therefore, the cooling point of this glass is given at about 980 ° C. from FIG. Meanwhile one of this glass
Coefficient of thermal expansion with respect to SiO _{2 at} 000 ° C., ie, (β _core− β
_{The cladding} is given by about 5.0 × 10 ^-4 from FIG. Therefore, when converted to a value at 980 ° C., (β _core− β
_{The cladding} is calculated in proportion (5.0 × 10 ^-4 ) × ｛(980-2
0) / (1000−20)｝ = 4.9 × 10 ⁻⁴ . S is about 0.012 mm when the cross-sectional area of the clad is sufficiently large compared to that of the core, and E is about 7000 kg / mm in SiO ₂ . Substituting the above into equation (1) gives t ≧ 41 g. In other words, it indicates that drawing should be performed with a drawing tension of 41 g or more. According to the above calculation, the base material was drawn with a drawing tension of 50 g, and a low-loss fiber of 0.205 dB / Km was obtained.

(Effects of the Invention) The present invention draws a base material having a large difference in thermal expansion between a core and a clad with a predetermined drawing tension as described above. There is an advantage that the fiber can be canceled out by the strain and an extremely low loss fiber can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the drawing tension and the average breaking tension of a dispersion-shifted fiber, and FIG. 2 shows the transmission loss frequency of the dispersion-shifted fiber under a constant drawing tension. FIG. 3 is a histogram showing the transmission loss frequency of the dispersion shifted fiber when the drawing tension is changed, FIG. 4 is an explanatory diagram showing an example of the refractive index distribution of the dispersion shifted fiber,
Figure 5 is a graph illustrating the relative refractive index difference with respect to the doping amount and purity of SiO ₂ GeO ₂ to SiO _2, FIG. 6 is a graph showing the anneal point of the doping amount of the same glass Goo ₂ to SiO _2, graph Figure 7 is showing the relative thermal expansion with respect to the doping amount and SiO ₂ of GeO ₂ to SiO _2. In the figure, 1: the refractive index of the first core, 2: the refractive index of the second core, and 3: the refractive index of the cladding.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Daiichiro Tanaka 1440 Mutsuzaki, Sakura-shi, Chiba Pref. Fujikura Electric Cable Co., Ltd. Sakura Plant (56) References −31328 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C03B 37/00-37/16

Claims (1)

(57) [Claims] (1) By adding a large amount of an additive such as germanium or phosphorus which increases the expansion coefficient of quartz glass,
An optical fiber preform having a quartz glass clad member containing an additive that changes the refractive index of quartz glass by at most about 0.2% around a core member made of quartz glass whose refractive index distribution is controlled. Is drawn with a drawing tension not less than the drawing tension represented by the following equation (1). t ≧ (β core−β clad) × E × S Formula (1) t: drawing tension β core: thermal expansion of core glass from normal temperature to slow cooling temperature β cladding: from normal temperature to slow cooling temperature of core glass Thermal expansion of clad glass E: Young's modulus of clad glass S: Cross-sectional area of clad glass