JPH02133333A - Production of optical fiber - Google Patents

Abstract

PURPOSE:To offset the thermal expansion of a core by the tensile stress at the time of drawing of a clad and to obtain a product having an extremely low loss by specifying linear tensile force at the time of drawing the base material for a quartz optical fiber contg. a relatively large amt. of Ge and P, etc., in the core. CONSTITUTION:The core member consisting of the quartz glass which is controlled in the refractive index distribution by adding a large amt. of the additives, such as Ge and P, to increase the coefft. of thermal expansion of the quartz glass. The base material for the optical fiber provided with the clad member consisting of the quartz glass contg. the additives at about the ratio at which the refractive index of the quartz glass is changed just by about 0.2% around the above-mentioned core member is drawn. The drawing is executed under the liner tensile force (t) expressed by the equation or above. In the equation, betacore is the coefft. of thermal expansion of the core glass from an ordinary temp. down to a slow cooling temp.; betaclad is the coefft. of thermal expansion of the clad glass from ordinary temp. down to the slow cooling temp. of the clad; E is the Young's modulus of the clad glass; S is likewise the sectional area.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention uses a quartz-based optical fiber base material containing a relatively large amount of additives such as germanium and phosphorus in the core.

It concerns a method of drawing lines with low loss.

(Prior Art) When obtaining an optical fiber such as a 1.55 μm transmission dispersion shifted fiber containing a relatively large amount of additives in the core by drawing it from a base material, conventionally, the drawing process was performed using a relatively high drawing tension. It was believed that transmission loss could be reduced by doing so.

(Problem to be solved by the invention) However, there is no clear guideline as to how much drawing tension should be used to draw the line (it is important to avoid using unnecessarily high drawing tension). When drawing, the mechanical strength of the optical fiber after drawing decreases, and even a slight tensile stress can lead to breakage.The relationship between drawing tension and mechanical strength is Although it is not necessarily uniquely determined as it is influenced by the heating conditions of the base material and the surrounding atmosphere, the results obtained when the drawing conditions are fixed are shown in FIG. 1. These are the results of a tensile test of 100 quartz-based dispersion-shifted fibers with an outer diameter of 125 um and a length of 20 m.
Regarding loss, as a result of drawing various 1.55 μm dispersion shifted fiber base materials, histograms as shown in FIGS. 2 and 3 were obtained. Figure 2 shows the loss power distribution when the drawing tension is fixed at a constant 35 g and the fiber is made, and Figure 3 shows the loss power distribution when the drawing tension is changed from 60 to 150 g and the fiber is made. It shows the distribution.

Therefore, 1.55 μm transmission originally indicates a loss wavelength at which the theoretical loss of silica-based glass fiber is low.
At high transmission losses, its value is halved. Considering this point, the results shown in Figure 2.3 are by no means satisfactory.

(Means for Solving the Problems) From the above points of view, the present invention provides a quartz glass whose refractive index distribution is controlled by adding a large amount of additives such as germanium and phosphorus that increase the coefficient of thermal expansion of quartz glass. An optical fiber base material in which a cladding member made of quartz glass is provided around a core member made of glass and contains additives to an extent that changes the refractive index of the silica glass by about 0.2% at the most is expressed by the following formula ( The present invention provides a method for manufacturing an optical fiber, characterized in that it is drawn at a drawing tension equal to or higher than the drawing tension shown in 1).

t≧(β −Ol XEXS formula +1) core
Kraft, t: wire drawing tension β: thermal expansion of core glass from room temperature to slow cooling temperature core tensile coefficient coefficient of thermal expansion of rad glass E: Young's modulus of clad glass S: cross-sectional area of clad glass The large amount of dopant added is at least 0.5% of the refractive index of quartz glass.
For example, when only GeO□ is added. , 5 wt% or more.

Also, the cladding member is pure silica glass or GeO□
Even if dopants such as , Pieces, F, and ct are added, the refractive index is 0.2 lower than that of silica glass.
%, and is intended for those in which the difference in thermal expansion coefficient between the core center member and the cladding member is at least 2x to-'.

(Function) With the above configuration, the difference in thermal expansion between the core and the cladding can be offset by the tensile strain generated in the cladding glass during drawing, thereby achieving extremely low loss in the resulting optical fiber. Can be done.

As a result of examining various dispersion-shifted fibers, it was found that the required drawing tension is determined by the amount of germanium added in the portion with the highest thermal expansion. Furthermore, it has been found that in this type of fiber containing germanium as the main dopant, the loss does not change at all above the value shown by equation (1). Although it is not always possible to provide a sufficient physical or chemical explanation for the relationship between wire drawing tension and transmission loss, it is currently estimated as follows. In other words, glass doped with dopants such as germanium inherently has fluctuations in its refractive index within its structure, and depending on the relationship between the magnitude of the fluctuation and the wavelength of light, this can result in so-called Leh scattering. . However, when tensile stress is applied to such glass, the fluctuations in the refractive index further increase because the photoelastic constants differ locally. However, in the compression direction, this fluctuation acts in a direction that is rather suppressed (therefore, in this case, no increase in loss occurs; therefore, the transmission loss of the fiber can remain stable at a drawing tension equal to or higher than equation +1).

When drawing the base material according to formula (1), the remaining problem is a decrease in the mechanical strength of the fiberglass due to high-tension drawing, but to prevent this, it is necessary to keep the drawing atmosphere clean. It is sufficient to take normal precautions such as reducing the level of dust near the glass surface and reducing the humidity of the atmosphere to prevent the extension of scratches that occur on the glass surface.

Furthermore, since the above formula (1) does not depend on the drawing speed, 800
Even with the recent increase in speeds such as m7 minutes, it can be handled without any problem.

(Example) In order to obtain a low-loss dispersion-shifted fiber having the refractive index distribution shown in FIG. 4, the drawing tension was determined using equation +1). In FIG. 4, 1 is a first core having a refractive index distribution that changes sharply in the radial direction, and its diameter is approximately 4 μm.
2 is an extrinsic 14 μm second core located around the first core and has a constant refractive index equal to the refractive index of the outermost portion of the first core. 3 is a cladding with an outer diameter of 125 μm located around the second core, and has a refractive index slightly lower than that of the second core. Specifically, the refractive index difference Δ between the first core 1 and the cladding 3 is 1.0%, the refractive index difference between the second core 2 and the cladding 3 is 0.13%, and this change in refractive index is It is given by adding Ge. Now, β
As mentioned above, Cocoa consists of the first and second parts and has a distribution of refractive index, but if we look at △=1.0%, the corresponding 5i02-GeOa glass The degree of Ge02a is 15 wt% from FIG. Therefore, the annealing point of this glass is given at about 980° C. from FIG. On the other hand, this glass one
Thermal expansion vs. SiO□ at 000°C, that is, (β
-β) is approximately 5. OX io-'core
Given by craft. Therefore, converting it to the value at 980℃ (β, A β = LP proportionality, (5, OXl0-')
x ((980-201/(1000-20))=4.
9 x 10-'. Also, S is approximately 0, assuming that the cross-sectional area of the cladding is sufficiently large compared to that of the core. OL
2 mm, E 1 isio, about 7000 k
Substituting g/I1m, which is 6 or more, into equation (1), t
≧41g. In other words, it shows that it is sufficient to draw the wire with a drawing tension of 41 g or more. Based on the above calculations, the base material was drawn with a drawing tension of 50g and the result was 0.
A low loss fiber of 205 dB/kra was obtained.

(Effects of the Invention) As described above, this invention draws a base material with a large difference in thermal expansion between the core and the cladding with a predetermined drawing tension. This has the advantage that it can be canceled out by distortion, thereby making it possible to obtain a fiber with extremely low loss.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the drawing tension and average breaking tension of a dispersion-shifted fiber, Figure 2 is a histogram showing the frequency of transmission loss in a dispersion-shifted fiber under a constant drawing tension, and Figure 3 is a graph showing the relationship between the drawing tension and the average breaking tension of a dispersion-shifted fiber. A histogram showing the frequency of transmission loss of a 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 shows the doping amount of GeO□ to SiO□ and pure SiO
A graph showing the relative refractive index difference with respect to □, Figure 6 is Sin
Figure 7 is a graph showing the doping amount of GoO□ to 5iOz and the annealing point of the same glass.
Graph showing relative thermal expansion versus O□. In the figure, ■ = refractive index of the first core, 2: refractive index of the second core, 3: refractive index of the cladding. Patent applicant Fujikura Electric Cable Co., Ltd. Agent Patent attorney Takeuchi Shusan I Zukori 1 Zhang O (o) 4! Loss (if, 57μWL) (d cod-) Should be 1 circle 1st place

Claims (1)

[Claims] (1) By adding a large amount of additives such as germanium and phosphorus that increase the expansion coefficient of silica glass, the refractive index distribution of silica glass is controlled. The method is characterized in that an optical fiber base material provided with a cladding member made of quartz glass containing an additive that changes by at most 0.2% is drawn with a drawing tension equal to or higher than the drawing tension expressed by the following formula (1). Method of manufacturing optical fiber. t≧(β_core_ar−β_core_rod)×E×S formula (
1) t: Wire drawing tension β Core: Coefficient of thermal expansion of the core glass from room temperature to slow cooling temperature β Clad: Coefficient of thermal expansion of cladding glass from room temperature to slow cooling temperature E: Young's modulus S of clad glass: Cross-sectional area of clad glass