JPH03211505A - optical fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an optical fiber used in a system that detects the status of not only a single point but also many points by applying stress to the optical fiber. In particular, it relates to optical fibers used in detection systems that utilize reflected light generated when the optical fiber is bent.

(Prior Art) As an optical fiber stress-applying sensor that detects the situation not only at a single point but at many points by applying stress to an optical fiber, there is a sensor as disclosed in Japanese Patent Application Laid-Open No. 63-266333. Flood detection sensor and JP-A-60-141
There are lightning detection sensors as shown in Publication No. 121.

FIG. 2 is an example of this. In the figure, 21 is an optical fiber,
22 is 0TDR (Optical Time Do
The main reflectometry (optical time domain reflectometry) measuring section, 23 and 24 are part of the sensor, and 25 is a CPU. As shown in FIG. 2, stresses such as bending and lateral pressure are applied to the optical fiber 21 on the sensor parts 23 and 24 depending on the amount to be detected, and the resulting optical transmission loss is measured by the 0TDR measuring section 22. To detect. ○The TDR waveform is as shown in Figure 3.
At position 33.34 corresponding to 3.24 etc., the third
A difference in loss occurs as shown in the figure. However, 35.3
As shown in 6, a dead zone occurs over a range of several tens of meters after this step. This dead zone occurs largely when a pulse wider than the timing to capture backscattered light is used, but even if a pulse narrower than the timing to capture the backscattered light is used, it is due to the disturbance of the optical waveguide mode that occurs at the place where stress is applied. The occurrence of dead zones is inevitable. For this reason, there was a problem that some of the many sensors could not be placed close to each other.

One possible method for solving this problem is to utilize reflected light generated by sharply bending an optical fiber. The reflected light due to this bending is 0TD
In the R waveform, as shown as 31°32 in Fig. 3,
It appears as a signal that is sharp and larger than the previous signal level. For example, by taking a signal when a part of the sensor is not operating as a reference and detecting a signal that is higher than that reference level, sensor operation can be detected even in a dead zone from a bent part. is possible, and it is possible to place a large number of sensors close together. Also,
As announced in Proceedings of the 50th Japan Society of Applied Physics Academic Conference (Autumn 1989) 29P-ZE-9, we first bend an optical fiber and observe changes in the state of the medium surrounding the bent part. Systems that detect by reflected light are also known.

However, in conventional devices of this kind, the optical fiber used is a normal single-mode optical fiber or multi-mode optical fiber, so the level of the signal waveform caused by reflection is small, making it difficult to obtain sufficient detection sensitivity. was difficult.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned circumstances, and it is possible to solve the problem by providing a reflecting portion in the cladding region that has the function of reflecting leakage light due to bending of the optical fiber. The object of the present invention is to provide an optical fiber that can obtain high detection sensitivity.

(Means for Solving the Problems) The present invention provides an optical fiber comprising a core portion and a cladding portion, in which the cladding portion does not substantially affect the light propagating through the core portion when the optical fiber is not bent. The device is characterized in that a reflective section is provided in the region of the section.

The reflecting section may be arranged concentrically with the core section.

By providing a boundary surface in which the refractive index changes in the cladding part, a reflecting part can be formed.

By incorporating a scatterer into the cladding part, a reflecting part can be formed.

Metal fine particles can be used as the scatterer.

At least one of Au, Ag, Cu, and Pt can be selected as the metal fine particles.

(Function) The phenomenon of reflection caused by bending an optical fiber occurs even in ordinary optical fibers, albeit very weakly.

This is thought to be because light leaked due to bending is reflected by the outermost layer of the optical fiber and returns to the core again.

Optical fibers are usually coated with an organic coating such as silicone resin or UV acrylate resin for protection, so the state of reflection varies greatly depending on the state of the coating and the physical properties of the coating. Furthermore, since reflection occurs at the outermost layer of the optical fiber that is far away from the core, the amount of reflected light that returns to the core is extremely small. Therefore, the reflected light signal observed using 0TDR is extremely weak.

The present invention provides an optical fiber consisting of a core portion and a cladding portion, in which a reflective portion is provided in a region of the cladding portion that does not substantially affect light propagating through the core portion when the optical fiber is not bent. Therefore, when the optical fiber is not bent, there is no influence from the reflective section.

When the optical fiber is bent, leaked light from the core caused by the bending is reflected by the reflection section and returns to the core. Therefore, due to the presence of the reflective section in the cladding section, the reflected light is not affected by the state of the coating on the optical fiber.

In order to return the reflected light to the core, it is desirable to place the part that causes reflection as close to the core as possible in the cladding part of the optical fiber, but if it is placed too close to the core, the optical fiber will not bend. Even in this state, it will affect the light propagating through the core, leading to increased loss in areas other than a portion of the sensor.

Reflection of light also occurs due to changes in the refractive index of the medium.
A boundary surface with a changing refractive index can be used as a reflective part.

Further, even when there is a scatterer such as a metal fine particle, the light is scattered and acts as a reflection part. Therefore, it is also possible to use a scatterer-containing layer as the reflection part.

(Example) FIG. 1 is a sectional view of an optical fiber for explaining an example of the present invention. In the figure, 11 is a core section, 12 is a cladding section, and 13 is a reflecting section. The reflecting part 13 is the core part 11
However, when the optical fiber is not bent for sensing, the core portion 11
is placed in a position where it does not substantially affect the light propagating through it.

When the optical fiber is bent, the light leaking from the core due to the bending is reflected by the reflection part 13, and part of the reflected light propagates through the core part 11 again, and part of it is converted into a 0TDR waveform as backscattered light. Appears as a signal light.

By arranging the reflective portion as a reflective layer concentric with the core portion as shown in FIG. 1, it is possible to provide flexibility in the bending direction of the optical fiber.

FIG. 4 is a cross-sectional view of an optical fiber for explaining another embodiment. In the figure, 11 is a core part, 12 is a clad part,
14 is a reflecting section. When the bending direction of the optical fiber is restricted to one direction, as in this embodiment, the reflecting section 1
4 does not necessarily have to be provided concentrically with respect to the core portion.

The reflective portion can be formed using a refractive index change interface. FIGS. 5(A) to 5(D) show examples of refractive index distribution structures when forming refractive index change interfaces,
Although the refractive index distribution is shown in a pattern with the vertical axis representing the refractive index and the horizontal axis representing the radius, the dimensions in the figure are not shown in proportion to the refractive index and radius. In FIGS. 5(A) to 5(D), the portion indicated by the arrow ↓ is the refractive index change boundary surface, and this boundary surface exists concentrically with the core portion. Although a multimode graded-index optical fiber is used as an example here, it is obvious that it can also be applied to step-index multimode fibers, single-mode optical fibers, etc.
In particular, the invention is not limited to the type of optical fiber.

When a scatterer is used as the reflective part, metal fine particles dispersed in a fiber medium can be used. In a silica-based optical fiber, these metal particles are A u + A g
Metals such as r Cu + P tend to aggregate in a colloidal form in quartz glass, making them easy to use.

Examples that further embody the present invention will be described below.

m-Example 1-1 As shown in the cross-sectional structure and refractive index distribution in Figure 6, the composition is G.
em□-3io2. An optical fiber consisting of a graded index core with a diameter of 50 μm and a relative refractive index difference △"=0.7% at the center of the core, and a cladding with a composition of SiO2y and a diameter of 125 μm. , F is added so that the relative refractive index difference is -0,
An optical fiber provided with a reflective layer of 6% and 5 μm thick was used.

The refractive index change boundary surface that has a reflective function is indicated by an arrow ↓.

In the figure, the refractive index at the position indicated as SiO2 is the refractive index of pure 5in2. As explained in FIG. 2, the reflected light generated when the optical fiber was bent was measured. In the 0TDR waveform (wavelength 1.3 μm) when the bending diameter is 25 mm, as shown in FIG. 7, in addition to a step difference of 0.7 dB due to bending, a sharp reflective waveform of about 0.2 dB was generated. In Figure 7, the bending was done at two locations, 300m and 700m from the end of the optical fiber, but as shown in Figure 8, the bending was done at 200m and 250m from the end of the optical fiber.
Even if they were placed close to each other at 0 m, the two reflected lights could be clearly separated.

m-Comparative Example 1 - Using an optical fiber having the same configuration as in Example 1 but without a reflective layer, the same observation of reflected light as in Example 1 was carried out. Although a very small amount of reflected light was observed, the signal waveform was 0.
．． The reflected light intensity was about 0.05 dB or less, and the intensity of the reflected light was too weak to be applied to an actual system.

m-Comparative Example 2-1 As shown in the cross-sectional structure and refractive index distribution in Figure 9, the composition is G.
A refractive index change layer was provided in the 65 μm diameter portion of the cladding portion of a graded index core with em2-8in□, diameter 50 μm, and relative refractive index difference △” = 0.7% at the center of the core.This refractive index change The layers are indicated by arrows ↓.The inner cladding part of this refractive index change layer is made of SiO□, and the outer cladding part has a refractive index lowered by 0.6% by adding F. In the figure, the refractive index at the position indicated as 5iOz is the refractive index of pure SiO□.This occurs when the optical fiber is bent as explained in Fig. 2 using this optical fiber. The reflected light was measured. In the 0TDR waveform when the bending diameter was 25 mm, the level difference due to bending was less than 0.1 dB, and almost no reflected light was observed. This means that up to the cladding part with a diameter of 65 μm is substantially the same as the core. However, by adjusting the incident conditions from the 0TDR so that the light does not propagate to the inner cladding, it is possible to use the 0TDR in this optical fiber and observe the reflected light due to bending. is considered possible.

m-Example 2-1 As shown in the cross-sectional structure and refractive index distribution in FIG. 10, the composition is Ge○23102. An optical fiber consisting of a graded index core with a diameter of 50 μm and a relative refractive index difference Δ”=0.3% at the center of the core, and a cladding with a composition F S The reflective part has a composition of SiO rate.

Using this optical fiber, as explained in Figure 2,
The reflected light generated when the optical fiber was bent was measured. 0TDR waveform when the bending diameter is 25mm (
1.3 μm), a level difference due to bending of 0.6 dB and a reflected light level of 0.2 dB, which is comparable to that of Example 1, were obtained.

-Example 3-1 As shown in the cross-sectional structure and refractive index distribution in Fig. 11, the composition is GeO□-3iO□, the diameter is 50 μm, and the relative refractive index difference at the center of the core is 0.3%. The index core has an inner cladding composition of F-3 with a diameter of up to 65 μm.
iO□, the composition of the outer cladding up to 125 μm in diameter is S
This is an optical fiber made of iO□, and the interface between the outer cladding and the inner cladding is used as a reflecting part. The refractive index change interface in the reflective part is indicated by an arrow ↓.

In addition, in the figure, the refractive index at the position indicated as 5iOz is the refractive index of pure S I O 2 . Using this optical fiber, as explained in FIG. 2, the reflected light generated when the optical fiber was bent was measured. The bending diameter is 25
In the 0TDR waveform (1.3 μm) when mmφ is,
A level difference due to bending of 0°8 dB and a reflected light level of 0.2 dB, which is comparable to that of Example 1, were obtained.

-Example 4-1 As shown in Fig. 12, the cross-sectional structure and refractive index distribution, a graded index core with a composition of GeO2S i 02 + diameter of 50 am + relative refractive index of 0.7% at the center was used. , using an optical fiber with a cladding having a composition of SiO□ and a diameter of 125 μm, in which Au fine particles are dispersed in a 5 μm thick portion from a diameter of 65 μm to a diameter of 75 μm in the cladding portion, and this is used as a reflective layer, Reflected light was measured as explained in FIG. As a result, in the 0TDR waveform (wavelength 1.3 μm) when the bending diameter is 25 mm, in addition to the step difference of 0.7 dB due to bending,
A sharp waveform due to reflected light of about 0.15 dB (which can also be said to be scattered light by the Au fine particle dispersion section) was observed. Note that the concentration of Au particles was estimated to be several 1100 pp.

In addition, this optical fiber is made by depositing a porous glass layer on a glass base material that corresponds to a portion of up to 65 μm in diameter.
A composite of a glass base material and a porous glass layer was immersed in a HA u C14 solution of 1/100 mole per liter, and the porous glass layer was impregnated with HAuCl and the solution, and then dried and heat treated. The base material for optical fiber is made by drawing a base material for an optical fiber obtained by making the base material transparent by depositing a porous glass layer on the outside again and making it transparent.

(Effects of the Invention) As is clear from the above explanation, by using the optical fiber according to the present invention, leakage light due to bending can be efficiently converted into reflected light and measured by 0TDR, and it is possible to measure not only a single point but also many points. This has the effect of providing an effective optical fiber for a point situation detection system.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view for explaining an optical fiber according to an embodiment of the present invention, FIG. 2 is a schematic diagram for explaining a system for detecting bending points using ○TDR, and FIG. 3 is a sectional view for explaining an optical fiber according to an embodiment of the present invention.
A line diagram of an example of an R waveform, FIG. 4 is a cross-sectional view for explaining an optical fiber of another embodiment, and FIG. 5 is an explanatory diagram of a refractive index distribution of an optical fiber of another embodiment of the present invention. , FIG. 6 and subsequent figures are for explaining specific examples. FIG. 6 is an explanatory diagram of the cross-sectional structure and refractive index distribution of the optical fiber in Example 1, and FIGS. , 0TD in Example 1
A diagram of the measurement results of the R waveform, FIG. 9 is an explanatory diagram of the cross-sectional structure and refractive index distribution of the optical fiber in Comparative Example 2, and FIG. 10 is a diagram of the cross-sectional structure and refractive index distribution of the optical fiber in Example 2. An explanatory diagram, FIG. 11 is an explanatory diagram of the cross-sectional structure and refractive index distribution of the optical fiber in Example 3, and FIG. 12 is an explanatory diagram of the cross-sectional structure and refractive index distribution of the optical fiber in Example 4. 11... Core part, 12... Clad part, 13.14
...Reflector.

Claims (1)

[Claims] In an optical fiber consisting of a core part and a cladding part, a reflective part is provided in a region of the cladding part that does not substantially affect the light propagating through the core part when the optical fiber is not bent. optical fiber.