JPS60237735A - Locating system for fault point - Google Patents

Abstract

PURPOSE:To allow a fault point locating circuit which detects reflected light of a light pulse sent out to an optical transmission line to locate the reflected light of the light pulse, and calculate the distance from a specified repeater to a fault point by providing each optical repeater with the fault point locating circuit. CONSTITUTION:Each optical repeater is provided with a photoelectric conversion part 20, identification and regeneration part 30, photoelectric conversion part 40, and fault point locating circuit 50. The conversion part 20 converts a light pulse from a terminal station or leading repeater into an electric signal, the regeneration part 30 identifies the pulse, and the conversion part 40 outputs a light pulse with a constant level. This light pulse is applied to optical switch circuits 51 and 52 of the circuit 50 and the circuits 51 and 52 are switched with a control signal identified by a control circuit 36. A light pulse from a light emitting element 43 is sent out by said switching to a transmission line through the switch 51, a directional coupler 53, and the switch 52. Backward scattered light of this transmission line is applied to and branched by the counter 53 by the switch 52 and converted by a photoelectric converting element 54 into an electric signal, which is processed by an averaging processing circuit 56.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a fault point search method that makes it possible to easily search for fault points between repeaters in an optical digital relay transmission system.

Conventional technology and problems When an optical fiber has a break point and a light pulse is input from the input end, Fresnel reflected light from the break point and backscattered Rayleigh scattered light that returns in the axial direction are incident. It will bring you back to the edge. Therefore, the breaking point can be detected using the Fresnel reflected light with respect to the incident light. However, the amount of Fresnel reflection is approximately 4% (-1
4 dB), and when seawater enters the break point when applied to a submarine optical fiber cable, the amount of Fresnel reflection becomes close to zero, making detection of the break point nearly impossible.

On the other hand, backscattered light is uniformly generated from all points on the optical fiber, but is not generated from beyond the break point, so it has been proposed to measure the backscattered light to detect the break point. For example, FIG. 1 is a block diagram of the main parts of a break point detection device based on backscattered light measurement, in which a modulation drive circuit 1 directly modulates and drives a laser diode 2 with a sine wave, and directs the output light of the laser diode 2 in the optical direction. The optical fiber is made to enter the optical fiber 9 to be measured via the optical coupler 3. This figure shows a case in which the terminal end of the optical fiber 9 to be measured is immersed in matching oil 10 to simulate a state in which seawater has entered the break point of the optical fiber.

In addition, the output light of the laser diode 2 is input into a half mirror, or one of the output lights output in both directions is inputted into the photodiode 4, the output signal is amplified by the amplifier 5, and the signal to be measured is split by the directional coupler 3. The backscattered light of the optical fiber 9 is made incident on the photodiode 6, its output signal is amplified by the amplifier 7, and the output signal of the amplifier 5.7 is applied to the gain and phase measuring circuit 8 to determine the frequency characteristics of the optical fiber 9 to be measured. The breaking point is detected by measuring and performing inverse Fourier transform.

Most of the light propagation loss in an optical fiber is due to Rayleigh scattered light caused by fluctuations in the refractive index inside the core, and the light that returns in the axial direction of this Rayleigh scattered light is backscattered light. The time axis response of this backscattered light is, for example, as shown in FIG.
This backscattered light is branched by the optical direction coupler 3 and is incident on the photodiode 6. If the power of this backscattered light is Pb, the incident light power is Pi, the backscattered light power is Rh, the group velocity of light in the optical fiber is V, and the optical fiber constant is α, then 0t≦T, Pb (tl= (Pi Rb v/ 2) exp'
(-cx v t )...(1) The backscattered light power Pb is expressed.

The frequency response of the backscattered light is given by Fourier transform of the waveform expressed by the above-mentioned equation (1). That is, it is expressed as exp(-+2πft)dt--+2).

In the frequency response of this backscattered light, t causes ripples in the gain and phase characteristics, and FIG. 3 shows an example of the frequency characteristics (gain) of this backscattered light.

In the figure, the horizontal axis is the frequency f, the vertical axis is the backscattered light power pb, Δr is the ripple size, and Δf is the ripple period. The period Δf of this ripple is defined as Δf
=v/2L (3) There is a relationship as follows. Therefore, by measuring this ripple period Δf, the break point of the optical fiber can be detected. In addition, the ripple size Δr is determined by the length of the optical fiber (
The ripple period Δf becomes smaller as the length of the optical fiber becomes longer. Also, as the frequency increases, the level of backscattered light decreases. The phase characteristic in the frequency domain is 4πfL when there is almost no Fresnel reflection.
A ripple occurs at a period of /V.

It has also been proposed to detect a breaking point by performing inverse Fourier transform processing on such frequency characteristics. This inverse Fourier transform is performed according to the following equation. That is, Pb (t)=2 J Pb (f) exp(+2
πf t) d f A characteristic of the backscattered light power pb corresponding to the distance #L to the break point of the optical fiber is obtained. As can be seen from FIG. 4, a plurality of peak points including the input end occur, but the maximum peak point excluding the input end indicates the length of the optical fiber. Therefore, when applied to submarine optical fiber cables, even if seawater enters the break point and Fresnel reflection is close to zero, the break point can be detected by detecting and processing the backscattered light. I can do it.

The gain phase measurement circuit 8 has a configuration that measures the frequency characteristics of the backscattered light described above and measures the distance from the ripple period Δf to the breaking point, or further performs inverse Fourier transform processing to determine the peak of the transformed output. It can be configured to measure the distance to the breaking point by points, for example, the sine wave frequency that modulates the laser diode 2 is set to 5'0.
OKHz, and measures the frequency characteristics of backscattered light every 10kHz, performs inverse Fourier transform processing on the measured data, calculates the maximum value from the processed data, and determines the distance at which the maximum value occurs as the breaking point. In this case, the measurement error of backscattered light gain is 0.1 dB, and the phase measurement error is 1 dB.
In the case of degrees, the measurement distance resolution was within ±15rn. Such a gain-phase measurement circuit 8 is configured by a microprocessor or the like, and can perform inverse Fourier transform processing through program processing.

The optical fiber break point detection method described above is OF
DR (Optical Frequency Dom)
This is called ainRef Iectometry (frequency domain optical reflection measurement), and is an effective method for detecting the break point of an optical fiber from an end station. In addition, in the optical analog relay system, it is possible to transfer backscattered light to the terminal station, so it is also possible to apply the 0FDR system, but in the optical digital relay transmission system, each Since the repeater regenerates and repeats the pulse waveform, it is impossible to repeat and transfer backscattered light. Therefore, it has been considered that the 0FDR method cannot be applied to the optical digital relay transmission method.

OBJECTS OF THE INVENTION It is an object of the present invention to apply the 0FDR method to an optical digital relay transmission method to enable accurate detection of a break point in an optical fiber between repeaters.

Structure of the Invention The present invention provides a failure point search circuit for detecting reflected light generated by an optical pulse sent to an optical fiber transmission line in each optical repeater in an optical digital repeater transmission system, and detects a carrier frequency from a terminal station. transmitting a modulated optical pulse, transmitting the optical pulse through each of the optical repeaters, and setting the failure point search circuit of the optical repeater designated by the terminal station to an operating state,
The reflected light from the relayed optical pulse is detected by the failure point search circuit, the detection information of the reflected light is transferred to the terminal station, the frequency characteristic of the reflected light with respect to the carrier frequency is measured, and the This method locates the distance from a designated repeater to a failure point, and an embodiment will be described in detail below.

Embodiment of the invention FIG. 5 is a block diagram of an optical digital relay transmission system according to an embodiment of the invention, in which 11 is a terminal station, 12-1, 12-2
．． . . . 12-1 is an optical repeater, 13 is a down optical fiber transmission line, 14 is an up optical fiber transmission line, and 15 is an intervening line or power line. Each optical repeater 12-1.12-2.
... 12-1 is provided with a failure point search circuit to be described later, and the optical repeaters are sequentially specified from the terminal station 11, or the optical repeater before the estimated failure point is specified, and the optical repeater is The fault point search circuit provided in the terminal station 11 is activated, and an optical pulse modulated at the carrier frequency is sent from the terminal station 11 to the downlink optical fiber transmission line 13.

For example, if optical repeater 12-2 is specified, terminal station 1
1 to 6 (a), the carrier frequency fc
The modulated optical pulse is sent to the down optical fiber transmission line 13. This optical pulse is relayed and transmitted by the optical repeater 12-1, and also relayed and transmitted by the optical repeater 12-2. The fault point search circuit of the optical repeater 12-2 detects, for example, the backscattered light shown in FIG. 6(b) as reflected light from the optical fiber transmission line. The detection information of this backscattered light is converted into a digital signal by encoding, and the intervening line or power line 15
The data is transferred to the terminal station 11 by. If the terminal station 11 measures the frequency characteristics and obtains, for example, the characteristics of the frequency f shown in FIG. L=v/(2Δf) It can be determined from (5).

FIG. 7 is a block diagram of the main parts of the optical repeater according to the embodiment of the present invention, in which 20 is a photoelectric conversion section, 3o is an equalization reproduction section, 40 is an electro-optic conversion section, 5o is a fault point search circuit, and 21 is a photoelectric conversion section. A photoelectric conversion element such as an avalanche photodiode, 22 a preamplifier circuit, 23 a bias control circuit, 31 an equalization amplifier circuit,
32 is an identification reproduction circuit, 33 is a timing extraction circuit, 34
is the peak i[4Jl road, 3 s HaAa cm increase m'r
sra, 41 is a pulse current drive circuit, 42 is a synthesis circuit,
43 is a light emitting element such as a laser diode, 44 is a photoelectric conversion element for monitoring the output light of the light emitting element 43, 45 is a comparison circuit, 46 is a reference voltage, and 47 is a bias current drive circuit.

Except for the failure point search circuit 50 (the configuration is that of a known optical repeater, the optical pulse from the terminal station 11 or the preceding optical repeater is converted into an electrical signal by the photoelectric conversion element 21 of the photoelectric conversion unit 20). and is amplified by the preamplifier circuit 22 and applied to the equalization reproducing section 30.In the equalizing reproducing section 30, the equalized amplified output signal amplified by the equalizing amplifier circuit 31 is identified and reproduced. Circuit 32. Added to the timing extraction circuit 33 and the peak voltage detection circuit 34. AG is added so that the peak value detected by the peak voltage detection circuit 34 is constant.
The equalization amplifier circuit 31 and the bias control circuit 23 are controlled from the C amplifier circuit 35. Further, the identification/reproduction circuit 32 performs pulse identification based on the timing signal extracted by the timing extraction circuit 33 and applies the identification/reproduction pulse to the pulse current drive circuit 41 of the electro-optical converter 40 . The extracted timing signal is also applied to the failure point search circuit 50.

In the electro-optical conversion section 40, the output light of the light emitting element 43 is detected by the photoelectric conversion element 44, and the comparison circuit 45 detects the output light from the reference voltage 46.
The comparison output signal is sent to the bias current drive circuit 47.
In addition, the bias current from the bias current drive circuit 47 and the pulse current from the pulse current drive circuit 41 are combined by a synthesis circuit 42 and applied to the light emitting element 43, and a constant level of light is generated by controlling the bias current. It is controlled so that pulses are output.

FIG. 8 is a block diagram of a fault point search circuit according to an embodiment of the present invention, in which 51 and 52 are optical switches that mechanically move or optically switch optical fibers, and 53 is an optical switch that controls optical direction. A coupler, 54 a photoelectric conversion element, 55 an amplifier, 5
6 is an averaging processing circuit, 57 is an encoding processing circuit, and 36 is a control circuit including the timing extraction circuit 33. When the optical switches 51 and 52 are switched in the direction of the solid arrow by the control signal from the control circuit 36, the light pulse output from the light emitting element 43 does not pass through the optical directional coupler 53.
It is sent out to an optical fiber transmission line.

In the optical repeater designated for fault detection, the fault detection circuit 50 is activated, and the fault detection circuit 50 is activated by the control signal transmitted via the control line or the optical digital signal transmitted via the optical fiber transmission line. The control signal is sent to the control circuit 3.
6, and switch the optical switches 51 and 52 as indicated by the dotted arrows. Thereby, the light pulses output from the light emitting element 43 are sent out to the optical fiber transmission line via the optical switch 51, the optical directional coupler 53, and the optical switch 52.

Backscattered light, which is reflected light from this optical fiber transmission line, is applied to an optical directional coupler 53 via an optical switch 52, and the branched backscattered light is applied to a photoelectric conversion element 54, where it is converted into an electrical signal. Ru.

This converted signal is amplified by an amplifier 55 and applied to an averaging processing circuit 56.

The optical pulses sent out from the terminal station 11 are modulated with a carrier frequency when searching for a fault, and the carrier frequency is changed within the passband of the bandpass filter of the timing extraction circuit 33. Since the optical pulse is sent out multiple times at predetermined time intervals, the backscattered light shown in FIG. 6(b), for example, is detected multiple times. Therefore, the backscattered light information is averaged by an averaging processing circuit 56, encoded by an encoding processing circuit 57, and transferred to the terminal station 11 as a digital signal via an intervening line or power line 15. The terminal station 11 processes this encoded backscattered light information to determine the frequency characteristic shown in TC in FIG. 6, and calculates the distance from the ripple period Δf to the breaking point. Furthermore, inverse Fourier transform is performed to determine the point of maximum value as the breaking point.

Therefore, in the optical digital relay transmission system, it becomes possible to easily and accurately detect the breaking point between repeaters.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, the present invention provides optical repeaters 12-1 . A failure point search circuit 50 is provided in li2, .
The terminal station 11 sends out an optical pulse modulated with a carrier frequency, the optical repeater relays and sends out this optical pulse, and the terminal station 11
The failure point search circuit 50 of the optical repeater designated by is activated, detects the reflected light of the optical fiber transmission line, transfers the detection information to the terminal station 11, measures the frequency characteristics with respect to the carrier frequency, This method locates the distance from a designated repeater to a fault point, and by using backscattered light, seawater, etc. can enter the break point of the optical fiber transmission line and the Fresnel reflection will be close to zero. However, the fracture point can be located with high accuracy. In addition, each optical repeater is only provided with a relatively simple failure point search circuit 50, and a specified optical Since the detection information detected by the fault point search circuit 50 of the repeater is transferred to the terminal station 11, there is an advantage that the fault point search at the terminal station can be performed with high accuracy.

[Brief explanation of the drawing]

Figure 1 shows block diagrams 1 and 2 of the 0FDR method measuring device.
Figure 3 is a time axis response characteristic curve diagram of backscattered light, Figure 3 is a frequency response characteristic curve diagram of backscattered light, Figure 4 is a curve diagram of the Fourier inverse transform processing result, and Figure 5 is a diagram of a curve diagram of the results of the Fourier inverse transform process. A block diagram of the main parts of the optical digital relay transmission system, FIG. 6 is an explanatory diagram of the failure point search operation, FIG. 7 is a block diagram of the main parts of the optical repeater according to the embodiment of the present invention, and FIG. 8 is a diagram showing the implementation of the present invention. FIG. 2 is a block diagram of main parts of an example failure point search circuit. 11 is a terminal station, 12-1.12-2. ...is an optical repeater,
15 is an intervening line or power supply line, 2o is a photoelectric conversion section, 30 is an identification/reproduction section, 4o is an electro-optical conversion section, and 5° is a fault point search circuit. Patent Applicant Fujitsu Ltd. Representative Patent Attorney Shoji Aitani Representative Patent Attorney Hiroshi Watanabe - Figure 1, Figure 2, Figure 3, Figure 4, 0, Mitsufu Iba Naga 3L, Figure 5, 1, and Figure 6

Claims (1)

[Claims] Each optical repeater in the optical digital relay transmission system is equipped with a failure point search circuit that detects the reflected light generated by the optical pulse sent to the optical fiber transmission line, and the optical pulse modulated at the carrier frequency is sent from the terminal station. Then, the optical pulse is relayed by each of the optical repeaters, and the fault point search circuit of the optical repeater designated by the terminal station is activated, and the reflected light from the relayed optical pulse is sent to the fault point. Detect it with a search circuit, transfer the detection information of the reflected light to the terminal station, measure the frequency characteristics of the reflected light with respect to the carrier frequency, and locate the distance from the specified repeater to the fault point. This is a failure point search method that is characterized by: