JPH04259813A - Method and device for evaluating strain distribution in optical cable - Google Patents

Abstract

PURPOSE:To easily recognize a change in the status of an optical cable after the cable is laid by making an optical pulse, the polarized state of which is adjusted, incident to the optical fiber of the optical cable and separating the back scattering light of the pulse into two orthogonally crossing polarized components, and the, photoelectrically detecting at least one of the polarized components. CONSTITUTION:When a local stress or geometric strain exist in an optical fiber 2 to be measured in an optical cable 1 to be evaluated, the light propagated through the stressed or strained section is polarized and modulated and its back scattering light enters a lens 15 from the fiber 2. After entering the lens 15, the light is made incident to a polarizing prism 16 after it is reflected by an opaque mirror 14 and spatially separated into two orthogonally crossing polarized components. The polarized components are respectively photoelectrically converted by means of photoreceptors 17A and 17B and the outputs of the photoreceptors 17A and 17B are averaged by means of a computer The processed results are displayed as the positional function the fiber 2 in the length direction. Therefore, when the intensity change in connection with the position of at least one of the polarized components of the back scattering light is observed, the position and extent of the strain applied to the optical fiber 2 can be estimated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical cable strain distribution evaluation method and apparatus for observing changes in the longitudinal condition of an optical fiber cable, which is a transmission medium for optical communication, after installation.

0002

BACKGROUND OF THE INVENTION Conventionally, as a device for evaluating the strain distribution in the longitudinal direction of an optical fiber, a measuring device using Brillouin scattered light has been known (T. Horiguch
i, T. Kurashima and M. Ta
teda, Trans, IEICE Japan,
J73-B-1, 144 pages, 1990).

0003

[Problems to be Solved by the Invention] However, the above-mentioned device requires expensive equipment such as two or more narrow-band light sources and an optical frequency analyzer, and also requires measurement from only one end of the optical fiber. There are drawbacks such as being impossible.

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide an optical cable strain distribution evaluation method and apparatus that can easily determine changes in the state of an optical fiber after installation by measuring one end of the optical fiber cable. .

[0005]

[Means for Solving the Problems] The optical cable strain distribution evaluation method according to the present invention achieves the above object, in which a light source that repeatedly oscillates optical pulses transmits light whose polarization state has been adjusted to an optical fiber in an optical fiber cable to be evaluated. Inject a pulse,
The optical cable strain distribution evaluation device according to the present invention is characterized in that the backscattered light emitted from the input end of the optical fiber is separated into two mutually orthogonal polarization components, and at least one of the polarization components is photoelectrically detected. A light source that repeatedly oscillates light pulses, and a light pulse from this light source that adjusts the polarization state and enters the optical fiber in the optical fiber cable to be evaluated, and generates backscattered light that exits from the input end of the optical fiber. An optical system that collects light, a light receiving system that separates the backscattered light collected by this optical system into two mutually orthogonal polarized components, and receives and photoelectrically detects at least one of the polarized light components, and this light receiving system. and a calculator that averages and displays the detected light intensity as a function of reception time.

[0006]

[Operation] When a local stress or geometric strain exists in the optical fiber to be measured in the optical fiber cable to be evaluated, the light propagating through that portion undergoes polarization modulation. At this time, when at least one of the two linearly polarized light components of the backscattered light from the optical fiber under test is detected independently, the observed light intensity will be accompanied by level fluctuations due to the influence of polarization modulation. . This polarization modulation is a function of the scattering position along the length of the optical fiber, because the change in polarization state is particularly noticeable in the strained portion of the optical fiber, and the frequency tends to increase. The degree and location of distortion can be estimated from changes in the received light intensity. Therefore, a light pulse is input into an optical fiber from a light source that repeatedly oscillates light pulses, and the backscattered light emitted from the input end is separated into two mutually orthogonal polarized components, at least one of which is photoelectrically detected. By averaging the intensity as a function of reception time, it is possible to estimate the degree of distortion and the location of the optical fiber.

[0007] Furthermore, when both of the two polarized light components are received and processed independently, the changes in the received light intensity with respect to the longitudinal position of the cable are complementary, and the combined sum of the two can be calculated using an ordinary optical pulse tester. The waveform reflects the same optical loss. Therefore, in this case, it is also possible to distinguish between intensity fluctuations due to polarization fluctuations and simple loss changes.

[0008] As a device for evaluating backscattered light, an optical pulse tester, OTDR (S.D.
ck, Bell Syst. Tech. J. ，
56, p. 355, 1977) has been known for a long time. However, conventionally, OTDR has been used to observe the intensity sum of all polarized light components of backscattered light, and only the optical loss value in the longitudinal direction of the optical cable can be obtained.

[0009]

EXAMPLES The present invention will be explained below based on examples.

FIG. 1 shows the configuration of an example of an apparatus for carrying out the method of the present invention. In the figure, 1 is an optical fiber cable to be evaluated, 2 is a single mode optical fiber held in the optical fiber cable 1, and 10 is an optical cable strain distribution evaluation device. The optical cable strain distribution evaluation device 10 has a light source 11 that oscillates a periodic optical pulse train.
The optical system for inputting the light from the light source 11 into the optical fiber 2 includes a 1/4 wavelength plate 12, a 1/2 wavelength plate 13, and a semi-transparent mirror 14 in this order. 4
After passing through the wavelength plate 12 , the half-wave plate 13 , and the semi-transparent mirror 14 , the light enters the optical fiber 2 via the lens 15 . Further, the backscattered light emitted from the optical fiber 2 enters the semi-transparent mirror 14 via the lens 15, is reflected, and enters the light receiving system. here,
The light receiving system includes a polarizing prism 16 such as a Wollaston prism, and light receivers 17A and 17B, and the two linearly polarized lights separated by the polarizing prism 16 enter the light receivers 17A and 17B, respectively, and are photoelectrically converted. . Furthermore, the optical cable strain distribution evaluation device 10 includes a light receiver 1
A computer 18 is provided which receives data on the received light intensity from 7A and 17B, averages the data, displays it, and controls the entire device.

Such an optical cable strain distribution evaluation device 10
A method of evaluating the strain distribution of the optical cable 1 using the following will be explained. From the light source 11, for example, the repetition frequency:
A light pulse of several kHz, peak value: tens of mW or more, width: 1 μm or less is emitted, and the optical fiber of the optical cable 1 is set to an appropriate polarization state by the quarter-wave plate 12 and half-wave plate 13. 2. Note that the appropriate polarization state is an incident polarization state in which the polarization fluctuation width of the backscattered light from the optical fiber 2 is the largest and the sensitivity is maximum, as will be described later.

The principle of the method of the present invention will now be explained with reference to FIG. As shown in FIG. 2, the light propagated through the optical fiber 2 held in a helical state within the optical fiber cable 1 is propagated while changing its polarization state. When applied locally, stress strain is induced at that location. As a result, the propagating light is modulated at that location, and the pitch of change in polarization state becomes shorter. In addition, since Rayleigh scattering in the optical fiber 2 preserves the polarization state during the scattering process, the backscattered light, which is the backscattered light, undergoes polarization modulation as much as it accompanies round-trip propagation to the scattering position. Return to the side. Note that the degree of polarization modulation at this time depends on the birefringence axis direction due to stress or geometric distortion in the optical fiber and the polarization angle of the optical pulse, so it is necessary to observe while adjusting the polarization state of the incident optical pulse. Accordingly, the degree of modulation received by the backscattered light can be adjusted to the optimum degree.

The backscattered light that has undergone polarization modulation and exits from the optical fiber 2 in this manner enters through the lens 15, is reflected by the semi-transparent mirror 14, and enters the polarizing prism 16, where it is spatially divided into two mutually orthogonal polarization components. and each light is photoelectrically converted by photoreceivers 17A and 17B. The outputs of the respective light receivers 17A and 17B are averaged by the computer 18 and displayed as a function of the scattering position in the optical fiber 2 to be measured, that is, the position in the longitudinal direction of the optical fiber 2.

An example of the output state at this time is shown in FIG. In Figure 3, (a) shows the case where the strain on the optical fiber 2 under test due to external stimulation is small, and (b) shows the case where the strain is applied locally (position Z1) due to bending, etc. The axis shows the position coordinate Z along the optical fiber 2, and the vertical axis shows the intensity I of backscattered light. And (a) and (b) are the waveforms after averaging processing of the outputs of the light receivers 17A and 17B, respectively.
(c) shows the processed waveform of the sum of the outputs of the two light receivers 17A and 17B.

As shown in FIG. 3, the actual optical fiber 2 originally has a slight birefringence that changes gradually along the longitudinal direction. b) shows a gradual fluctuation complementary to the position Z. If strain is applied locally (position Z1) due to factors such as bending or pulling, for example, the optical fiber 2
When the bending diameter is reduced, the birefringence value of that part increases almost in proportion to the degree of reduction, and as a result, as shown in (b), the fluctuation period of the received light intensity level partially decreases at position Z1. becomes shorter.

Therefore, by observing the intensity change with respect to the position Z of at least one polarization component of the backscattered light, the position and degree of strain applied to the optical fiber 2 can be estimated.

Note that the resolution of the position Z is twice the optical pulse width as in a normal optical pulse tester. In addition, the result of the output sum of the two light receivers 17A and 17B is shown in (c) in Fig. 3.
This is equivalent to the loss distribution waveform for position Z, which is similar to that of a normal optical pulse tester that does not separate polarization components.

Generally, the birefringence value along the optical fiber 2 changes over time, but if the state of the optical fiber cable 1 is stable, the period of change is relatively slow, on the order of several tens of minutes or more. In the present invention, there is no problem as long as the averaging processing time of the light receiver output is sufficiently shorter than this fluctuation period. To monitor temporal polarization fluctuation fluctuations, Fig. 3(a), (
In b), check whether the level of the Fresnel reflected light at the far end of the optical fiber propagating back and forth over the entire length of the optical fiber to be measured, seen in the protrusion (d) at the right end of the observed waveform, is stable within the measurement time. If this level is stable, the effects of fluctuations can be ignored.

FIG. 4 shows an optical cable strain distribution evaluation device according to another embodiment. As shown in the figure, this device is shown in Figure 1.
This is a simplified version of the light receiving system of the device shown in FIG. 1, and members having the same functions as those in FIG.

Optical cable strain distribution evaluation device 10 shown in FIG.
A shows a light receiving system in which a linear polarizer 19 and a light receiver 17C are arranged. That is, the backscattered light reflected by the semi-transparent mirror 14 enters the linear polarizer 19, and only one polarized light component enters the light receiver 17C, and the light intensity of this polarized light component is calculated by the computer 18. By performing the averaging process, the strain distribution of the optical fiber 2 can be estimated as described above.

Furthermore, the optical cable strain distribution evaluation device 1 shown in FIG.
At 0A, the linear polarizer 19 can rotate its main axis by 90 degrees and can also slide in a direction intersecting the direction of incidence of backscattered light. Therefore, by rotating the main axis of the linear polarizer 19 by 90 degrees, the other polarized light component can be incident on the light receiver 18C and its intensity can be determined. Moreover, when the linear polarizer 19 is slid and moved out of the path of the backscattered light, the backscattered light is directly transmitted to the light receiver 1.
8C, and the intensity of the sum of the two polarized components can be determined.

[0022]

[Effects of the Invention] As explained above, according to the present invention,
Distortion in the longitudinal direction of an optical fiber and its changes, which have been difficult to evaluate in the past, can be easily estimated from one end of the optical fiber. Therefore, it becomes possible to observe the strain added to the optical fiber core of an optical fiber cable, especially changes in the amount of strain in areas where failures are relatively easy to occur, such as connection points and excess length after installation, and This has the effect that it is possible to monitor the condition of the fiber core and to obtain guidelines for renewal.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical cable strain distribution evaluation device according to an embodiment.

FIG. 2 is an explanatory diagram showing changes in polarization as propagation of a single mode optical fiber in an optical cable.

FIG. 3 is a graph showing a signal waveform after averaging processing of the light receiver output according to the example.

FIG. 4 is a configuration diagram of an optical cable strain distribution evaluation device according to another embodiment.

[Explanation of symbols]

1 Optical cable 2 Optical fiber 10, 10A Optical cable strain distribution evaluation device 11 Light source 12 1/4 wavelength plate 13 1/2 wavelength plate 14 Semi-transparent mirror 16 Polarizing prism 17A, 17B, 17C Light receiver 18 Computer

Claims (2)

[Claims] Claim 1: An optical pulse whose polarization state has been adjusted is input from a light source that repeatedly oscillates optical pulses into an optical fiber in an optical fiber cable to be evaluated, and the backscattered lights emitted from the input end of the optical fiber are orthogonal to each other. An optical cable strain distribution evaluation method characterized in that the optical cable is separated into two polarized light components, and at least one of the polarized light components is photoelectrically detected. [Claim 2] A light source that repeatedly oscillates light pulses;
an optical system that adjusts the polarization state of the light pulse from this light source, enters the optical fiber in the optical fiber cable to be evaluated, and condenses the backscattered light that exits from the input end of the optical fiber; A light receiving system that separates the collected backscattered light into two mutually orthogonal polarized components and receives at least one of the polarized light components for photoelectric detection, and a light receiving system that converts the light intensity detected by this light receiving system into a function of reception time. 1. An optical cable strain distribution evaluation device, comprising: a computer that performs averaging processing and displays the results.