JPH0136050B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the concentration distribution of OH groups in an optical fiber.

Conventionally, the concentration distribution of OH groups in an optical fiber has been measured by a method of measuring strong light absorption by OH groups at a wavelength of 2.73 μm using an infrared spectrometer. In this method, a SiC lamp is used as a light source, and a monochromatic light of 2.73 μm, which is the fundamental absorption wavelength of OH groups, is obtained using a diffraction grating spectrometer. and,
After converting this light into a parallel beam using a CaF ₂ lens system,
The light is applied perpendicularly to the measurement sample, which has been cut into a 1 mm thick disc and polished. The light flux that has passed through the measurement sample is captured by a low-OH multimode step optical fiber (about 30 cm long) with a core diameter of about 60 μm, guided to an InSb photodetector, where the detected signal is amplified by current and then input to a lock-in amplifier. do. When measuring the OH group distribution in the radial direction of the fiber base material, a sample moving stage is used to move the measurement sample in the direction perpendicular to the incident light flux.
The transmitted light intensity distribution is measured while moving at 50 μm intervals, and the OH group concentration distribution is determined from this. The disadvantages of this method are that it is difficult to measure minute OH groups of 0.1 ppm or less, and that there is a disturbance in the light transmission intensity distribution due to the rapid change in the refractive index at the boundary between the core and the cladding. Examples include:

An object of the present invention is to eliminate the above-mentioned drawbacks and utilize an optical fiber transmission loss measurement system that has the feature that measurement accuracy improves in proportion to the length of the optical fiber.
The absorption loss due to OH groups is determined, and the concentration distribution of OH groups in the optical fiber is derived.
The object of the present invention is to propose a method that can perform measurements of groups.

The invention will be explained in detail below with reference to the drawings.

In the present invention, single mode optical fibers having various standardized waveguide widths (or standardized frequencies) (hereinafter referred to as V values) manufactured from the same base material are used. An example of a wire drawing device is shown in FIG. In Figure 1,
1 is a base material for optical fiber, 2 is a resistance heating furnace, 3 is a fiber outer diameter measuring section, 4 is a coating device, 5 is a capstan, 6 is an optical fiber, 7 is a winding device, 8 is a fiber outer diameter control circuit, 9 is a base material feeding device. As is clear from FIG. 1, the optical fiber preform 1 is gradually fed to the resistance heating furnace 2 by the preform feeder 9. The tip of the base material heated to about 2000° C. in the resistance heating furnace 2 is softened and its viscosity is reduced to become a constricted fiber and drawn out by the capstan 5.
The drawn fiber 6 is wound onto a drum attached to a winding device 7. In addition, optical fiber 6
A coating device 4 coats the glass surface of the fiber 6 with urethane and silicon to maintain its strength. To control the outer diameter of the optical fiber 6, a fiber outer diameter measuring section 3 reads the outer diameter of the fiber 6, and a fiber outer diameter control circuit 8 compares the measured value with a set value to control the winding speed of the fiber. This is done by doing this.

Next, one embodiment of the present invention will be described. Outer diameter 12
A base material 1 with a synthetic cladding diameter of 5 mm, a core diameter of 0.7 mm, and a refractive index difference of 0.2% is mounted on the base material feeding device 9,
Approximately 500 m of fiber 6 having an outer diameter of 100 μm is produced by heating and melting by the above-described method at a drawing speed of 27 m/min. Next, set the winding speed to 23 m/min, and
Manufacture 500m of 120μm fiber 6. By repeating this process, the outer diameter of the fiber becomes
Six fibers are produced varying in 20 μm steps from 100 μm to 200 μm. The fiber produced in this way has a core diameter of about 5 μm to 10 μm, and has an OH
The V value at 1.38 μm, which is the original absorption loss wavelength, is designed to be 2.4 or less. Here, V=2π/λa√ ₁ ² − ₂ ² (1) where 2a is the core diameter, n ₁ is the refractive index of the core, n ₂ is the refractive index of the cladding, and λ is the wavelength. And V
Since <2.4, the only mode that propagates is the fundamental mode.

Next, we will measure the absorption loss due to OH groups of these fibers and the refractive index distribution of the core. FIG. 2 shows the absorption loss [dB/Km] due to OH groups at a wavelength of 1.38 μm for various fibers drawn by the above-mentioned method versus the V value.
Moreover, FIG. 3 shows the refractive index distribution of the core of the fiber mentioned above, and FIG. 4 shows the power distribution P of the fundamental mode at the distance r/a in this fiber.
(r) is shown for various V values.

Absorption loss due to OH groups (Figure 2) and core refractive index distribution (Figure 3) can be easily measured using existing measurement equipment. Also, the power distribution of the fundamental mode P
(r) (FIG. 4) is calculated from the refractive index distribution of the core. Therefore, the concentration distribution of OH groups in the fiber can be derived from FIGS. 2 and 4 by the following calculation.

The concentration distribution of OH groups in the cross section of the optical fiber is determined by the process of manufacturing the base material, and the drawing process has little effect on the concentration distribution of OH groups. Therefore, in optical fibers having various V values obtained from the same base material, the concentration distribution of OH groups in the optical fiber cross section will be similar in each optical fiber. That is, if the distance in the radial direction of each optical fiber is normalized based on the core diameter, the OH group concentration distribution in each optical fiber will be the same. Furthermore, as described in this embodiment, when changing the core diameter in order to obtain optical fibers with various V values, the outer diameter of the optical fiber is also changed at the same rate as the rate of change of the core diameter. Therefore, when the fiber cross section is divided at equal intervals in the radial direction, if the number of divisions is the same, the concentration of OH groups in each divided region will be the same for optical fibers having various V values.

First, the cross section of the fiber is divided into equal intervals in the radial direction. The number of divisions is 10. The concentrations of OH groups to be determined in each region are assumed to be C ₁ , C ₂ , ..., C ₁₀ . If the total power of the HE ₁₁ mode in the fiber cross section when V = 2.0 is normalized to 1, the power in each region can be found from Figure 4, and the values can be expressed as P _1,1 , P _{2, 1} ,...,P _10,1 . The absorption loss due to the OH group when V=2.0 can be determined from FIG. 2, and if that value is _l1 , then the following equation holds true for V=2.0.

Q×(C ₁ P _1,1 +C ₂ P _2,1 +…+C ₁₀ P _10,1 )=l ₁ (2) However, Q is a proportionality constant, and at a wavelength of 1.38 μm, Q=65. Here, Q gives the absorption loss value due to OH groups per 1 ppm, and the OH generated at 1.38 μm
The original absorption peak results in a loss value of 65 dB/km.
Similarly, for various V values (V<2.4), the following formula Q×(C ₁ P _1,2 +C ₂ P _2,2 +…+C ₁₀ P _10,2 )=l ₂ Q×(C ₁ P _1,3 +C ₂ P _2,3 +…+C ₁₀ P _10,3 )=l ₃ 〓 Q×(C ₁ P _1,10 +C ₂ P _2,10 +…+C ₁₀ P _10,10 )= l ₁₀ holds, so from the 10 rows and 10 columns of simultaneous equations,
The OH group concentrations C ₁ , C ₂ , ..., C ₁₀ can be derived. However, the number of divisions is not limited to 10, and the fibers can be applied to all fibers other than those mentioned above. The absorption wavelength of the OH group used is not limited to 1.38μm,
Wavelengths such as 0.94 μm and 1.24 μm can also be used.

Figure 5 shows the fibers derived in this way.
This shows the OH concentration distribution. From Figure 5,
It can be seen that according to the present invention, precise measurement of minute OH concentrations of about 10 ppb is possible. Note that since absorption by OH groups is very strong at 1.38 μm, measurement of 10 ppb or less is fully possible.

As explained above, in the optical fiber of the present invention,
According to the measurement method of OH group concentration distribution, the absorption loss value of OH group is used as data, but the measurement accuracy of such absorption loss value improves in proportion to the optical fiber length, so it is difficult to detect minute OH group concentration. It has the advantage that it is possible to measure Further, by feeding back the OH group concentration distribution measured by the method of the present invention to the optical fiber manufacturing process and controlling that process, it is possible to contribute to lowering the OH of the layer.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of a drawing apparatus for carrying out the present invention, Fig. 2 is a graph showing the V value dependence of absorption loss due to OH groups, Fig. 3 is a graph showing the refractive index distribution of the core, and Fig. 4 is a graph showing the V value dependence of absorption loss due to OH groups. The figure shows the fundamental mode power distribution V
A graph showing the values as parameters, FIG. 5 is a graph showing the derived OH concentration distribution in the fiber. 1...Fiber base material, 2...Resistance heating furnace, 3
...Fiber outer diameter measuring section, 4...Coating device, 5...
... Capstan, 6 ... Fiber, 7 ... Winding device, 8 ... Fiber outer diameter control circuit, 9 ... Base material feeding device.

Claims (1)

[Claims] 1 OH in optical fiber, which is a transmission medium for optical communication
In the method for measuring the concentration distribution of OH groups, calculations are made based on the measured values of absorption loss due to OH groups and the measured values of the refractive index distribution for single mode optical fibers with various standardized waveguide widths made from the same base material. A method for measuring the OH group concentration distribution in an optical fiber, characterized in that the OH group concentration distribution is determined by creating an equation with the OH group concentration as an unknown quantity using the calculated power distribution and solving the equation.