JPS6330739A - Method for measuring back scattering of optical fiber - Google Patents

Abstract

PURPOSE:To judge the presence of the bending radiation loss of an optical fiber and the bending position of said fiber, by a method wherein two or more kinds of lights mutually different in a wavelength are incident to the optical fiber and the difference between transmission losses obtained with respect to respective wavelengths is calculated. CONSTITUTION:A light source 1 has a light source 11 of a wavelength lambda1 and a light source 12 of a wavelength lambda2 and the lights 21 from the light source 1 pass through a transmission mirror 3 to be incident to an optical fiber 7 to be measured from the end surface thereof. The reflected lights 22 from the optical fiber 7 due to back scattering are detected by detectors 41, 42 corresponding to the wavelength thereof and subjected to data processing in an operational processing part 5 to be displayed on a display 6. When the plot of the difference between transmission loss coefficients of the optical fiber calculated by differentiation operation becomes a predetermined value or less, it can be judged that bending radiation loss is generated and, from the plot position to the graph thereof, a bending generating position is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical fiber backscattering measurement method for determining whether a bend exists in an optical fiber and the location of the bend.

<Prior Art> Optical fibers that have been widely used for communications in recent years have some degree of transmission loss. This transmission loss α(λ) is a value depending on the wavelength λ of the transmitted light, and can be expressed as the following 2.

α(λ) −A −+B+C(λ)+D+E(λ)...
・(1) λ 4 Here, A: Rayleigh scattering coefficient B: Structural imperfection loss C (λ): Absorption loss due to OH- groups and transition metals, etc. D = Microbending ignition E (λ): Bending radiation Loss By the way, microbending loss caused by stress due to the coating layer of the optical fiber, lateral pressure applied from above the coating layer, etc. is the loss due to the wood quality of the optical fiber, but the bending loss (λ) is the loss caused by the optical fiber being small, such as when a local bend occurs when winding the optical fiber onto a bobbin. This is the loss caused by bending to the radius. Therefore, the bending radiation loss E(
The defectiveness of the optical fiber should not be determined based on the transmission loss α(λ) including It can be used for any purpose.

Conventionally, bending radiation loss E(λ) has been determined by combining two methods: wavelength characteristics of transmission loss and backscattering measurement using one type of light. According to the former, as shown in FIG. 4, the characteristics when the optical fiber is not bent (a in the figure) are compared with the characteristics when the optical fiber is bent (b in the figure), Bending radiation loss E(
λ) can be determined. However, with this method it is not possible to determine where in the longitudinal direction the optical fiber is bent.On the other hand, according to the latter method, the transmission loss coefficient (dB/kn+), which is the differential value of the scattered light intensity with respect to the optical fiber length,
), the bending position can be determined from the wavelength 1.311
As shown in Figure 5, which shows the backscattered light intensity when light No. 11 is incident on the optical fiber, the bending position is measured from the position where the slope of the scattered light intensity (transmission loss coefficient) changes (point C in the figure). can be determined. However, with this method, it cannot be determined whether the change in the transmission loss coefficient is due to bending or not.

<Problems to be solved by the invention> Conventionally, the bending radiation loss E(
The presence or absence of λ) and its bending position were determined, but there were problems in that different devices were required for each measurement, and the work was complicated and took a long time.

The present invention has been made in view of the above-mentioned conventional circumstances, and an object of the present invention is to provide a backscattering measurement method that rationally solves the above-mentioned problems.

A method for measuring backscattering of an optical fiber according to the present invention involves injecting light into an optical fiber, and calculating the transmission loss of the optical fiber from the differential value of the intensity of the backscattered light with respect to the length of the optical fiber. In an optical fiber backscattering measurement method that measures changes in the longitudinal direction of
Two or more types of light having different wavelengths are incident on the optical fiber, the difference in transmission loss obtained for each wavelength is determined, and the presence or absence of bending radiation loss in the optical fiber and its bending position are determined from the difference in transmission loss. It is characterized by making a judgment.

<Example> An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a schematic configuration of a multi-wavelength backscattering measuring device that implements the method of the present invention. The light source 1 includes a light a11 with a wavelength λ1 and a light source 12 with a wavelength λ2, and the light 21 from the light source 1
is incident on the optical fiber 7 to be measured from the end face through the transmission mirror 3. The reflected light 22 from the optical fiber 7 due to back scattering is transmitted to detectors 41 and 42 according to its wavelength.
After being detected by and processed by the arithmetic processing unit 5,
displayed on the display 6. Here, the light a11 and the light 21 from the light source 12 may be modulated at different frequencies and may enter the optical fiber 7 at the same time, or may be time-divisionally entered using an optical or mechanical branching method. Similarly, the detectors 41 and 42 can simultaneously receive the reflected light 22 and perform frequency classification processing, or can detect the light source in a time-division manner using an optical or mechanical branching method. When the wavelengths of the light source 11 and the light source 12 are close, it is possible to use one detector and use a frequency discrimination method.

Next, the evaluation principle of radiation loss E(λ) due to bending will be explained.

In the wavelength range where normal optical fibers are used, absorption loss C(λ
) is so small that it can be almost ignored, and
Considering the dependence of the Rayleigh scattering coefficient A, structural imperfection loss B, microbending loss 01 and bending radiation loss E on the longitudinal distance from the input end face of the optical fiber 7 to be measured, the above (1) can be obtained. The formula is the following formula (2).

■ α(λ,L) = A(L) −+B(L)+D(L)
+E (λ, L) λ 4 (2) Here, α (λ, L): wavelength λ and distance M from the incident end surface
Transmission loss of the optical fiber at L A(L): Rayleigh scattering coefficient (μ4・dB/+a) at distance ML from the input end face B(L): Structural imperfection loss D(at distance #L from the input end face) L): Microbending loss E(λ, L) at a distance NIL from the input end face Bending radiation loss at two wavelengths λ and a distance jillL from the entrance end face In the above equation (2), two types of wavelength λ1 and wavelength λ2 are By measuring the backscattering with light and taking the difference between the two, we get the following (
3) Equation is obtained.

α(λ,L)−α(λ2.shi)・・・・・・(3) By the way, the Rayleigh scattering coefficient A(L) mainly depends on the composition of the glass, and the material of the optical fiber. If the glass composition is the same, the Rayleigh scattering coefficient A (L) will hardly change. For example, if the craft is SiO□,
The Rayleigh scattering coefficient of a single mode optical fiber whose core is made of GeOt and SiO□ and whose refractive index difference between the core and cladding is 0.27% to 0.33% is in the range of 0.91 to 0.97. Therefore, there is no bending radiation loss E(λ, L) and the wavelength λ1. λ2 is 1.3I amount, 1.55-
In this case, the following relationship is obtained from equation (3).

0.16 (dB/km)≦(a (1,3,L) −
a (1,55,L) 't≦0.17 (dB/km)
......(4) Here, the bending radiation loss E (λ, L) is λ1 less λ2, then E
There is a relationship of (λI,L) <E (λ2.L). Therefore, from this relationship and equations (3) and (42), if there is bending radiation loss E(λ, L), α(1,3,L)−cx
(1,55,L) < 0.16 (dB/km)
The relationship holds true.

Next, a specific example of a method for determining the presence or absence of bending radiation loss E(λ, L) and its bending position will be described.

In the graph shown in FIG. 2, curves d and e indicate the measured values of the backscattered light intensity when light with a wavelength of 1.3/II and 1.55μ is incident, respectively. Also, in this figure, the plot f is derived from the optical fiber length 1 from the curves d and e.
It shows the difference in the transmission loss coefficient of the optical fiber calculated every 00 m. That is, as shown in the figure, since the value of f is approximately O,l 6 (dB/km) or more, this optical fiber has a bending radiation loss E(λ.

It can be determined that L) has not occurred.

On the other hand, in the graph shown in FIG. 3, the curve d1. ,
e/ is wavelength 1.3 All, 1.55 respectively
The plot r/ shows the difference in the transmission loss coefficient of the optical fiber, which was differentially calculated in the same way as above. That is, as shown in the figure, f at position g
' value is 0.16 dB/km or less, which indicates that bending radiation loss E (λ, L) has occurred and that bending has occurred at this position g.

<Effects of the Invention> According to the present invention, the bending radiation loss of an optical fiber and its bending position can be determined in a short time and with ease using one type of measuring device, and the optical fiber can be used effectively. can.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a backscattering measurement device implementing an embodiment of the present invention, FIGS. 2 and 3 are graphs each representing a measurement example according to an embodiment of the present invention, and FIG. FIG. 5 is a graph showing the wavelength characteristic of fiber loss, and FIG. 5 is a graph showing the backscattered intensity of the optical fiber. In the drawings, 1, 11 and 12 are light sources, 41 and 42 are detectors, 5 is an arithmetic processing unit, 6 is a display, and 7 is an optical fiber.

Claims (1)

[Claims] In an optical fiber backscattering measurement method in which light is incident on an optical fiber and the longitudinal change in transmission loss of the optical fiber is measured from the differential value of the backscattered light intensity with respect to the optical fiber length, the optical fibers are The method is characterized in that two or more different types of light are incident, the difference in transmission loss obtained for each wavelength is determined, and the presence or absence of bending radiation loss in the optical fiber and its bending position are determined from the difference in transmission loss. Optical fiber backscatter measurement method.