JPH0464029A - Inspecting method for hermetic coat optical fiber - Google Patents

Abstract

PURPOSE:To easily detect an insufficient carbon coat layer by a method wherein an optical fiber subjected to hermetic coating is placed in an ambience of hydrogen, an optical pulse having the absorption wavelength of hydrogen is let in from one end and, the scattering echo is monitored from the entering side of the pulse. CONSTITUTION:An optical pulse from an LD light source 1 is let into an optical fiber 4 subjected to hermetic coating via a condenser lens 2 and a directional coupler 3. A minute scattering light is scattered rearward when the optical pulse advances in the optical fiber 4. The light of the scattering light which returns back to the entering end is received by a photodetector 6 through the directional coupler 3 and measured. Accordingly, the state of loss of the optical fiber 4 due to hydrogen is measured. In order to enhance the measuring sensitivity, response waveforms of a plurality of optical pulses are taken into an electronic circuit and averaged. In this manner, an insufficient part of the carbon coat layer can be easily detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for easily inspecting the coating state of an optical fiber provided with a hermetic coat such as carbon.

[Problems to be solved by conventional techniques and inventions] Currently,
The mainstream of optical fibers used for optical communications are those whose main component is quartz glass.

There are two problems with the long-term lifespan of this type of optical fiber.

One is the problem of long-term mechanical strength deterioration. This is because when tensile stress is applied to a glass fiber in the presence of humidity (moisture) in the atmosphere, the moisture and glass chemically react, producing silanol and causing scratches in quartz fibers, for example. There is a phenomenon in which it expands and eventually breaks.

Another problem is that of hydrogen ingress into the glass. To actually use optical fiber, it must be made into a cable that is easy to use. At this time, a small amount of hydrogen may be generated from the cable material, or hydrogen may be generated as a result of rusting of the metal used as the cable material. For example, silica glass has a very high permeability to light gases such as hydrogen and helium, and if a silica fiber with a 125-chain outer diameter is exposed to a hydrogen atmosphere, it will pass through the cladding into the core in a few hours. Hydrogen reaches up to It is known that when hydrogen reaches the core, its absorption loss affects wavelengths used for optical communications, and hydrogen also reacts with glass to generate 5iOH, leading to increased transmission loss at other wavelengths. .

Specifically, 1, which is the wavelength used for optical communication,
3 fall, 1.55-, the absorption wavelength by hydrogen molecules is 1.24, and the absorption wavelength when hydrogen is converted to an OH group is 1.3! bm and 1.24a++, other absorption wavelengths such as 1.52- should be taken into consideration.

In reality, these absorption wavelengths are not single wavelengths but have a certain spectral width, so the tail of the spectrum affects the wavelength used in optical communications.

FIG. 4 is a loss wavelength characteristic diagram of a silica-based glass fiber due to H2, showing this situation. In the figure, ■ indicates the results before the test, and ■ indicates the results after the test.

A method that is attracting attention as a way to solve these problems is to coat the glass surface with a hermetic coat such as amorphous carbon. The carbon is several hundred angstroms thick, which is extremely thin compared to the standard profile of the fiber. By coating with amorphous carbon, increase in mechanical strength and loss due to hydrogen can be prevented. However, there has been no method to check whether carbon is properly coated over the entire length of the optical fiber.

[Means to solve the problem]

The present invention provides a method for inspecting the coating state of a hermetic coated fiber by combining a hydrogen permeation test and an optical pulse echo observation from the above viewpoints. The purpose is to inspect the coating state of the barmetal coating by placing a \ in a hydrogen atmosphere, injecting a light pulse having a wavelength absorbed by hydrogen from one end, and observing the scattered echoes on the incident side.

Here, the absorption wavelength by hydrogen refers to 1,24n, which is the absorption wavelength by hydrogen molecules, 1,39, and 1.24jrrn, which are the wavelengths when hydrogen is converted to an OH group.

Furthermore, the hermetic coated fibers to be tested include not only carbon coated fibers but also metal coated fibers, TiC coated fibers, and the like.

[Effect]

When a hermetic coated optical fiber is placed in a hydrogen atmosphere, hydrogen will enter the optical fiber from any portion where the coating is insufficient. Therefore, the wavelength 1.2, which is the absorption wavelength of hydrogen molecules or OH groups, is
When a light pulse of 4 or 1.39μ is sent from one end of an optical fiber, absorption of the light pulse occurs at the point where hydrogen enters, so by observing the scattered echoes of the light pulse, it is possible to detect insufficient hermetic coating. The parts are revealed.

〔Example〕

Figure 1 shows the optical pulse echo observation [optical pulse test or OT D R (optical
time domain reflectometer
y) is an explanatory diagram.

In the figure, 1 is the absorption wavelength by hydrogen, 1, 24 μ
Or an LD light source that generates a 1.39n light pulse,
2 is a condensing lens, 3 is a directional coupler consisting of an optical coupler, etc., 4 is a carbon coated fiber wound on a reel and housed in a hydrogen atmosphere container 5, one end of which is connected via a connector etc. It is connected to one output end of the directional coupler 3, and the other end is open. 6 is a photodetector;
It is connected to the other output end of the directional coupler 3.

In the above configuration, a light pulse from the LD light source 1 is input into the optical fiber 4 via the directional coupler 3. When the light pulse travels into the optical fiber 4, there is weak scattered light that is scattered backwards. Of this scattered light, the light that returns to the incident end is transmitted to a photodetector 6 via a directional coupler 3.
By measuring this, the loss state due to hydrogen in the optical fiber can be measured. In order to increase sensitivity, measurements are performed by inputting response waveforms to multiple optical pulses into an electronic circuit and averaging them. If the carbon coat covering the optical fiber is uniform, dense, and of excellent quality, the second
As shown by A in the figure, a linearly decreasing waveform output is obtained. In the figure, the horizontal axis is distance (converted from time), and the vertical axis is output.

If there is an insufficient carbon coating covering the optical fiber, hydrogen will enter the optical fiber from that area, resulting in a light pulse of 1, 24n or 1.39 wavelengths being sent, which is the hydrogen absorption wavelength. is absorbed, and the output of the returning scattered light drops there, creating a step, as shown by B in Figure 2.

Thus, by measuring the distance to the position where the step occurs, the location where the carbon coating is insufficient can be found. Furthermore, the extent of the defect can be determined by measuring the size of the step.

(Experiment example) As the optical fiber to be tested, approximately 750 ml of amorphous carbon was coated on a silica-based glass fiber with a diameter of 125 Ifrn.
A material coated with a thickness of angstroms was prepared, and before being wound onto a reel, the state of carbon coating was intentionally made to be uneven in two places before winding it up. This fiber was housed in a container, and the inside of the container was made into a hydrogen atmosphere of 1 atmosphere. A light pulse with a wavelength of 1.24 degrees was used as a light source. Immediately after the start of the test, there was no particular abnormality in the output, and the waveform appearing in the output of the optical pulse tester was almost linear as shown by A in Figure 3, with an average loss (slope of the straight line) of 0.7 dB.
/km. However, after one hour, as shown in the figure, a step difference (increase in loss) was gradually observed in two areas where the carbon coating was incomplete. After that, this step gradually became larger, and after 24 hours, the maximum loss (slope) reached about 4 dB/km, as seen in C. In addition, when we measured the positions where the steps occurred, they were 900 m and 2600 m from the fiber end on the measuring instrument side (resolution approximately 30 m), and the actual measurement positions were 920 m and 2600 m.
At 20m, we were able to observe the anomalous point almost accurately without much difference. Further, the thickness of the carbon coat at the location where the maximum loss (inclination) was about 4 dB/km was measured and found to be about 150 angstroms, which was about 175 angstroms originally.

〔Effect of the invention〕

As described above, the method of this invention uses the 01DR method to detect an increase in loss due to hydrogen intrusion into the inside of the fiber when the hermetink-coated fiber is placed in a hydrogen atmosphere. can be detected.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the optical pulse echo observation method used in the method of this invention, Fig. 2 is a graph showing the observation results obtained by the method of Fig. 1, and Fig. 3 shows the experimental results according to the invention. The graph shown in FIG. 4 is a loss wavelength characteristic diagram of a silica-based fiber due to H2. In the figure, 1: LD light source, 3 bidirectional couplers, 4: carbon coated film, 5: container, 6: photodetector. Figure 1 B

Claims (1)

[Claims] 1. Place a hermetic coated optical fiber in a hydrogen atmosphere, enter a light pulse with a wavelength absorbed by hydrogen from one end, and inspect the state of the hermetic coat by observing the scattered echoes on the incident side. A method for inspecting a hermetic coated optical fiber.