JPH02176535A - Optical line monitoring device - Google Patents

Abstract

PURPOSE:To confirm a loss in overall length of an optical fiber line and to execute preventive maintenance monitoring by executing an optical time domain reflectometry (OTDR) measurement by light beams of two wavelength. CONSTITUTION:Pulse light beams of wavelength lambda1, lambda2 are generated from light sources 1, 2, driven 3, coupled 4, and thereafter, made incident on a fiber line to be measured 6 through a branch device 5. Subsequently, a reaction light returned to the line 6 passes through the branch device 5 and led to a demultiplexer 7, demultiplexed to wavelength lambda1, lambda2, and sent to light receivers 8, 9. Outputs of these light receivers 8, 9 are sent to a signal processing circuit 10, and a time variation of a reflected light output is derived. Accordingly, a minute loss variation can be measured by a light beam from the light source 1, and monitoring extending over the overall length of the line 6 can be executed by a light beam from the light source 2.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an optical line monitoring device for monitoring the condition of an optical fiber line from a distance.

[Prior art 1] In order to monitor the condition of optical fiber lines from a distance, 0TDR (0 optical time-doma) has conventionally been used.
Monitoring devices based on the inReflectmetry method are widely used. This method involves injecting a light pulse into one end of the optical fiber line to be measured, detecting the reflected light that returns to that end, and capturing the time change in the output of the reflected light. This method attempts to determine the light level, and this allows measurement of loss at each location of the optical fiber line and monitoring of the coupling state. [Problems to be Solved by the Invention] However, monitoring of optical fiber lines using the conventional 0TDR method is limited to simply checking the quality of the communication state or checking the loss of the line, and it is difficult to determine what the line condition will be in the future. The problem is that it is difficult to perform preventive maintenance as things change. In other words, the normal 0TDR method cannot measure loss changes with high sensitivity, so it cannot measure minute changes caused by changes in lateral pressure on the line due to ground subsidence or collapse of conduits, twisting of optical fibers, etc. It is from. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical line monitoring device that can monitor the line condition from a distance and also perform highly sensitive monitoring for preventive line maintenance.

[Means to solve the problem]

In order to achieve the above object, the optical line monitoring device according to the present invention includes a first and a second light source that generate optical pulses of different wavelengths and enter the optical fiber line to be measured; The light receiver is equipped with a light receiver that receives the reflected light that has propagated at each of the wavelengths, and a circuit that measures time changes in the output of the reflected light for each of the wavelengths. It is characterized in that the wavelength is shorter than and near the cutoff wavelength of the second mode of the one-mode optical fiber, and the wavelength of the light from the second light source is set away from the cutoff wavelength.

[Function] On the shorter wavelength side than the cutoff wavelength of a single mode optical fiber, the second mode propagates out into the cladding, and at wavelengths near the cutoff wavelength, even a slight bend causes a large radiation loss. On the other hand, at wavelengths far from the cutoff wavelength, the radiation loss is small, so backscattered light from a long distance can be measured. Therefore, the first, which generates optical pulses of these two wavelengths,
By using the second light source and measuring the reflected light of these lights using the oTDR method, it is possible to measure minute loss changes due to the light from the first light source, and this can be used to detect small changes in loss due to land subsidence or collapse of pipelines, etc. Changes in lateral pressure applied to the track, twists in optical fibers, etc. can be detected, making preventive maintenance of the track possible. On the other hand, since the measurement using the light from the first light source has extremely high sensitivity as described above, if there is even a slight loss increasing factor along the way, it becomes impossible to measure at a further distance. Therefore, the entire length of the long-distance optical fiber line is monitored by using the light from the second light source. By performing 0TDR measurements using a two-wavelength light source in this way, it is possible to both monitor the line for preventive maintenance through highly sensitive measurements, and to confirm the quality of the communication status over the entire length through long-distance measurements. be able to.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an optical path monitoring device 20 according to an embodiment of the present invention, which is equipped with two laser diodes 1.2 having different oscillation wavelengths, which are pulse-driven by a pulse drive circuit 3. , generates pulsed light. These pulsed lights of wavelengths λ1 and λ2 are sent to the coupler 4.
After being combined and wavelength-multiplexed, the signals are input to one end of the optical fiber 6 to be measured via the splitter 5. In addition to the coupler 4, a diffraction grating, a geodesic lens, etc. can also be used in order to wavelength-multiplex the pulsed lights of wavelengths λ1 and ^2 in this way. The reflected light returning to one end of the optical fiber line 6 to be measured passes through a splitter 5 and is guided to a splitter 7 such as a diffraction grating, where it is split into light with wavelengths λ1 and λ2, and each is received. Sent to vessel 8.9. The outputs of these light receivers 8, 9 are sent to a signal processing circuit 10, and a time change in reflected light output is determined. The control circuit 11 sends control signals such as timing signals to the pulse drive circuit 3 and the signal processing circuit 10. 0TD using such optical pulses with two wavelengths ^1 and λ2
With the configuration R, backscattered light due to Rayleigh scattering of the optical fiber line 6 to be measured is measured. In this example,
As the optical fiber line 6 to be measured, a single mode optical fiber having an outer diameter of 125 μm, a core diameter of 10 μm, and a relative refractive index difference of 0.28% was coated with silicone to have an outer diameter of 400 μm. In the case of this single mode optical fiber, the second mode cutoff wavelength was 1.18-. Therefore, the first
The oscillation wavelength of the second laser diode 1 was 0.9 μm, and the oscillation wavelength of the second laser diode 2 was 0.85 μm. Since this wavelength of 0.9 μm is on the shorter wavelength side than the cutoff wavelength and in the vicinity thereof, a large loss due to bending appears. Therefore, by using the pulsed laser of this wavelength from the laser da-fode 1 to capture the loss change due to Rayleigh scattering at a wavelength of 0.9 μm using the 0TDR method, we can also detect slight changes in strain in the optical fiber line 6 to be measured. It was possible to detect with high sensitivity. For example, the single mode optical fibers (length 200 m) forming the optical fiber line 6 to be measured are wound into a coil shape with a diameter of 300 m and bundled,
When lateral pressure was applied to this, a loss change of 2 dB or more appeared, which could be detected. In addition, this single mode optical fiber was twisted at 1/16 pitch (22 twists per 5 m length).
．． When a 5° twist was applied), a loss change of 1.6 dB appeared, and this could be detected. On the other hand, in the case of a pulse laser with a wavelength of 0.85 μm from the second laser diode 2, the wavelength is far from the cutoff wavelength, so the radiation loss does not appear large, and the optical fiber line under test is We were able to detect only a large change in the loss of the single mode optical fiber, which is 6. In addition, the cutoff wavelength is 1.1 μm to 1.2 μm in commonly used single-single doped optical fiber for wavelength 1.3 μm.
On the shorter wavelength side than this wavelength and 0.8
Since the loss due to bending is large in a wavelength range longer than μm, the wavelength (the oscillation wavelength of the laser diode 1 in the above embodiment) for detecting a fault point with high sensitivity needs to be within this range. On the other hand, the other wavelength (the oscillation wavelength of laser diode 2 in the above embodiment) is the normal 0T.
Similar to DR, since it is intended to capture only the large loss over the entire length of the optical fiber line under test, the wavelength is set farther from the cutoff wavelength than the first wavelength above, and the wavelength range where the loss due to bending is smaller is set. Must be set. In the embodiments described above, the cutoff wavelength was set to the shorter wavelength side, but it can also be set to the longer wavelength side. Further, since the cutoff wavelength differs somewhat depending on the single mode optical fiber forming the optical fiber line 6 to be measured, it is desirable to change the two wavelengths used accordingly. FIG. 2 shows the optical line monitoring device 2o of FIG.
This shows an example of the installation in 1. As shown in this figure, in a telephone office 21, an optical fiber line 22 laid outside is connected to a relay system 23 via a distribution circuit panel 24, and an optical line monitoring device 20 is coupled to this.

【Effect of the invention】

According to the optical line monitoring device of the present invention, by performing 0TDR measurement using light of two wavelengths, in addition to confirming loss along the entire length of the optical fiber line, it is possible to search for fault points with extremely high sensitivity, and to perform preventive maintenance. Monitoring is possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an optical line monitoring device according to an embodiment of the present invention, and FIG. 2 is a block diagram showing an example in which the optical line monitoring device of FIG. 1 is installed in a telephone office. 1.2...Light source, 3...Pulse drive circuit, 4...
Coupler, 5... Brancher, 6... Optical fiber line to be measured, 7... Brancher, 8.9... Light receiver, 1o...
Signal processing circuit, 11... Control circuit, 20... Optical line monitoring device, 21... Telephone office, 22... Optical fiber line, 23... Relay system, 24... Distribution circuit panel.

Claims (1)

[Claims] (1) First and second light sources that generate optical pulses with different wavelengths and enter the optical fiber line to be measured, and receive reflected light that has propagated through the optical fiber line to be measured for each of the wavelengths mentioned above. It has a light receiver and a circuit that measures the time change of the reflected light output for each wavelength, and the wavelength of the light from the first light source is set to a wavelength shorter than the cutoff wavelength of the second mode of the single mode optical fiber. An optical path monitoring device characterized in that the wavelength of the light from the second light source is set to a wavelength far from the cutoff wavelength.