JPH06350531A - Method and device for detecting optical input interruption - Google Patents

Abstract

(57) [Summary] [Object] To provide a simple and accurate optical input disconnection detection method for an optical input disconnection detection method in an optical communication system that performs a plurality of communications using a single optical transmission line. To do. [Structure] Each original signal is converted into a plurality of signals having different average levels and transmitted, and the average level of the reception signal electrically converted in the receiver is determined.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of detecting an interruption of an optical signal, and more particularly, to an optical signal effective in a system for performing optical bidirectional transmission or optical wavelength division multiplexing transmission using a single optical transmission line. The present invention relates to a method for accurately detecting disconnection of the.

In an optical backbone transmission line, a pair of optical fibers are usually used to transmit an upstream signal and a downstream signal, but in an optical subscriber line or a private optical transmission line, one optical fiber is used for upstream transmission. In many cases, a bidirectional transmission method for transmitting a downstream signal and a wavelength multiplexing transmission method for multiplexing a plurality of signals by multiplexing lights of different wavelengths are used.

[0003]

2. Description of the Related Art FIG. 5 shows the configuration of an optical bidirectional transmission system. In FIG. 5, 11 is an optical transmitter A, 12 is an optical receiver A, 13 is an optical transmitter B, 14 is an optical receiver B, 1
5a and 15b are optical signal coupling / branching means, 17 is an optical transmission line, and 18 is an optical cable. In this system, the optical signal A is transmitted and received between the optical transmitting means A and the optical receiving means A, and the optical signal B is transmitted and received between the optical transmitting means B and the optical receiving means B. After each signal is sent from the optical transmitting means, it is placed on the optical transmission line by the optical signal coupling / branching means, branched by the optical coupling / branching means on the other side, and input to the optical receiving means. Therefore, the optical signals A and B propagate in opposite directions in one optical transmission line.

FIG. 7 shows a schematic diagram of the optical signal coupling / branching means. This port is connected to the optical transmitting means A, the port is connected to the optical receiving means B, and the port is connected to the optical transmission line.
In the case of the directional coupler type optical signal coupling / branching means, the loss from port to port and the loss from port to port are small, and if the loss from port to port is large, the signals A and B interfere. You can communicate without doing.
In the case of the wavelength multiplexing type optical signal coupling / branching means, the loss for the wavelength of the signal A between the ports is small and the loss for the wavelength of the signal B between the ports is small. Of the signal B from the port A to the port A
If the loss for the wavelength of is large and the loss for the wavelengths of the signals A and B between the ports is large,
The signals A and B can communicate without interfering with each other.

FIG. 6 shows the configuration of an optical multiplex transmission system.
In FIG. 6, 11 is an optical transmitting means A, 12 is an optical receiving means A, 13 is an optical transmitting means B, 14 is an optical receiving means B, 16a.
And 16b are optical signal multiplexing / demultiplexing means, 17 is an optical transmission line 18
Is an optical cable. In this system, the optical signal A is transmitted and received between the optical transmitting means A and the optical receiving means A, and the optical signal B is transmitted and received between the optical transmitting means B and the optical receiving means B. After each signal is sent out from the optical transmitting means, it is wavelength-multiplexed by the optical signal multiplexing / demultiplexing means and placed on the optical transmission line, and is separated into each wavelength by the optical signal multiplexing / demultiplexing means on the other side, and the optical receiving means Entered in. Therefore, the optical signals A and B having different wavelengths propagate in the same direction through one optical transmission line.

FIG. 7 also serves as a schematic diagram of the optical signal multiplexing / separating means. This port is connected to the optical transmission means A, the port is connected to the optical transmission means B, and the port is connected to the optical transmission line.
Then, the loss for the wavelength of the signal A between the ports is small, the loss for the wavelength of the signal B between the ports is small, the loss for the wavelength of signal B from port to port is large, and the loss from port to port is large. If the loss of the signal A for the wavelength is large and the loss of the signals A and B between the ports for the wavelength is large, the signals A and B can communicate without interfering with each other.

As described above, the transmission efficiency of the optical subscriber line and the local transmission line is improved. However, in the above-described optical bidirectional transmission or optical multiplex transmission system, a problem occurs when the optical transmission line or the like is disconnected.

FIG. 8 shows a problem when the optical transmission line is disconnected in the optical bidirectional transmission system. When the optical transmission line is cut as shown in FIG. 8, the optical signal A is reflected at the cutting point, propagates backward through the optical transmission line, and is input to the optical receiving means B. As described above, since the loss from the optical transmission means A to the optical transmission line and the loss from the optical transmission line to the optical reception means B are small, the optical signal A is at a level attenuated by the return loss at the cut point. Is input to the light receiving means B.

Since the optical receiving means B detects the input interruption by monitoring the level of the input optical signal and the level of the reproduced timing component, the input interruption is not detected when a certain level is inputted. Since the return loss when the optical transmission line is disconnected is 14 dB at best, the optical receiving means B
The light level input to is attenuated by only about 14 dB at maximum. At such a level, the optical receiving means B does not detect the input interruption, and receives the optical signal A which is not the signal to be originally input.

FIG. 9 shows a problem in the optical multiplex transmission system when the optical cable from one optical transmitting means to the optical signal multiplexing / separating means is cut. When the optical cable from the optical transmitting means A to the optical multiplexing / demultiplexing means is cut as shown in FIG. 9, only the optical signal propagating through the optical transmission line becomes B. This is supplied from the receiving side optical signal multiplexing / separating means to the optical receiving means B and becomes a received signal. At this time, the wavelength component of the optical signal B also leaks into the port of the optical signal multiplexing / separating means on the receiving means A side. This amount of attenuation is normally secured only to the extent that there is no problem in the transmission of the main signal, so that the receiving means B does not receive the signal A to be received.
Does not detect input interruption.

Therefore, in any of the methods, there is a problem in that the input means cannot detect the input interruption even though the optical transmission line or the optical cable is cut and the normal signal is not inputted.

[0012] Conventionally, as a method for solving this, there has been proposed a method of notifying that the signal is a normal signal by utilizing an overhead portion of a transmission signal format. In this method, the optical receiving means which is a physical interface determines the signal. In addition, if an abnormal signal is input when the optical transmission line or optical cable is disconnected as described above, the receiving level is low and errors easily occur in the receiving means. Since it is highly probable that accurate judgment is difficult.

[0013]

SUMMARY OF THE INVENTION The present invention addresses such a problem and accurately detects that an optical transmission line or optical cable is cut and a signal to be originally received is not input. Another object of the present invention is to provide a method for detecting an input break in optical multiplex transmission.

Specifically, the nature of the signal input to the receiving means is directly determined.

[0015]

FIG. 1 is a waveform diagram for explaining the principle of the present invention. In FIG. 1, 1 is the original signal, 2
Is a signal-1 obtained by code conversion of the original signal, and 2 is a signal-2 obtained by code conversion of the original signal. Signal-1 is the so-called Retu
rn to Zero (RZ) signal. Signal-2
Is a Return to One signal that sets "0" for only a half time slot to the logic level "0" and fixes the logic level to "1" at other times. Then, when the signal -1 is added to the optical signal A, the optical signal B
The communication is performed by using signals having different characteristics, such as carrying signal-2 on.

Next, the second principle of the present invention will be described. The second principle is to distinguish signals by using a technique of premodulating an original optical signal with a low frequency signal in order to suppress stimulated Brillouin scattering in an optical fiber transmission line.

[0017]

The average level of signal-1 and signal-2 in FIG. 1 is expressed by the following equation. Average level = 0.5 × (mark ratio) + 0.5 × (fixed logic level value) Here, the fixed logic level value is 0 for the signal-1 and 1 for the signal-2. Now, in a practical communication system, the code is always scrambled in order to prevent the disappearance of the timing component due to the continuation of the same code, so that the mark ratio is about 0.5 even in a short time average. Therefore, the average level of signal-1 is about 0.25, and signal-
The average level of 2 is about 0.75. By determining this average level as a threshold of 0.5, it is possible to determine whether or not the received signal is a regular signal.

The second principle is as follows. In the optical receiver, in order to control the electric converted signal to a predetermined level, an amplifier for feeding back the signal obtained by peak detection of the output electric signal is used. If a low frequency signal for pre-modulation is detected from the signal obtained by detecting the peak and judgment is made, it can be judged whether or not the signal is a normal input signal.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows the structure of receiving means in an embodiment of the present invention. In FIG. 2, 120 is a light receiving element, 121
Is a received main signal amplifier, 122 is a peak detection circuit, 123
Is an automatic gain control circuit, 124 is an average level detection circuit, 1
Reference numeral 25 is a comparator, 126 is an identification circuit, 127 is a timing extraction circuit with a signal disconnection detection function, and 128 is an OR circuit.

The received signal is photo-electrically converted by the light receiving element,
It is amplified by the main signal amplifier. The output is applied to the discrimination circuit and the timing extraction circuit, and the extracted timing signal is discriminated on the basis of time to obtain the original signal. At the same time, the output signal of the main signal amplifier is guided to the peak detection circuit and the average level detection circuit, and the peak detection output controls the gain of the main signal amplifier. On the other hand, the voltage corresponding to the average value of the signal detected by the average level detection circuit is input to one terminal of the comparator and compared with the reference voltage corresponding to 0.5 described above. Now, if the correct received signal is signal -1,
If the output of the average level detection circuit is applied to the common mode input terminal of the comparator, the output level of the comparator is “0” when the signal −1 is input, and the comparison is performed when the signal −2 is input. The output level of the container becomes "1". Therefore, since this level is supplied to the logical sum circuit, the input disconnection can be detected. still,
The timing detection circuit is provided with a conventional signal disconnection detection circuit, and its output is also applied to the OR circuit.

Now, while various techniques are progressing in optical communication, the purification of the output spectrum of the semiconductor laser and the enhancement of the level of the input to the optical fiber by the optical fiber amplifier are recent great achievements. . However, since the core of an optical fiber is an extremely narrow space, various non-linear effects appear when a high level optical signal is input, and the transmission distance and input power have come to be restricted. Among them, a method of premodulating an original light signal with a low frequency signal has been proposed as a method of reducing stimulated Brillouin scattering that appears remarkably for light with high coherency.

The second embodiment of the present invention utilizes this low frequency pre-modulation signal to detect a break in the input optical signal. FIG. 3 is a diagram showing the configuration of the receiving means in the second embodiment of the present invention.

In FIG. 3, 120 is a light receiving element, and 121
Is a main signal amplifier, 122 is a peak detection circuit, 123 is an automatic gain control circuit, 126 is an identification circuit, 127 is a timing extraction circuit with a signal break detection function, 128 is an OR circuit,
Reference numeral 129 is a superimposed signal determination circuit.

The received signal is photo-electrically converted by the light receiving element,
It is amplified by the main signal amplifier. The output is applied to the discrimination circuit and the timing extraction circuit, and the extracted timing signal is discriminated on the basis of time to obtain the original signal. At the same time, the output signal of the main signal amplifier is guided to the peak detection circuit, and the peak detection output controls the gain of the main signal amplifier.

FIG. 4 is a waveform of each part in the circuit of FIG. The circled numbers annexed to the waveforms in FIG. 4 match the circled numbers annexed in FIG. 3, indicating that each waveform is the waveform of the points circled in FIG. There is. The waveform of the optical signal input to the light receiving element is Actually, since the optical signal having a constant intensity is pre-modulated at a low frequency and then the intensity is modulated by the communication signal, the optical input has a waveform interrupted by the communication signal, but since it is irrelevant to the essence of this embodiment, The state of intensity modulation by is not shown.
At any rate, the optical input is in the form of a low frequency signal superimposed on the original optical signal. The output electric signal of the light receiving element has a waveform similar to that of, and this is input to the main signal amplifier whose gain is automatically controlled by the signal obtained by peak detection of the output signal.
That is, since the main signal amplifier is fed back so that the peak has a predetermined level, the output signal thereof, that is, the signal at the discrimination point has a constant level as shown by. The voltage supplied from the automatic gain control circuit to the main signal must have the component of the low frequency signal in order that the waveform whose peak fluctuates with the low frequency signal becomes similar to the above. That is, the output signal of the peak detection circuit has a waveform in which such a low frequency signal is superimposed.

Therefore, the superposed signal determination circuit 129 is connected to the output terminal of the peak detection circuit 122 to detect the low frequency superposed signal, and whether the low frequency signal should be superposed on the regular optical signal or not. By determining whether or not the disconnection of the optical input signal can be detected. Specifically, when a sine wave is used as the low frequency signal, a filter can be provided and the output level can be used for the determination. When a digital signal is used, the pattern can be used for the determination. You can
In this way, the disconnection is detected by the output that determines whether or not the superimposed signal is a regular signal, and a disconnection alarm is issued. A conventional signal disconnection detection circuit is provided in the timing detection circuit, and its output is also applied to the OR circuit to issue an input disconnection alarm.

As described above, the optical input break detection can be performed by either of the configurations of FIGS. 2 and 3, but the configuration of FIG. 3 is advantageous when the number of signals passing through the common optical transmission line is large. . This is because in the configuration of FIG. 2, when the number of signals increases, the voltage difference to be discriminated becomes smaller, and a discrimination error is likely to occur, whereas in the configuration of FIG. 3, the signal is discriminated by discrimination of sinusoidal frequency or discrimination of code pattern. Since it suffices to determine
This is because the accuracy of discrimination can be increased.

[0028]

As described in detail above, in the optical bidirectional transmission or the optical multiplex transmission, by making a difference in the average level of each signal or premodulating each original optical signal with a low frequency signal, It becomes possible to detect the disconnection of the optical transmission line or the optical cable easily and accurately.

As a result, it becomes unnecessary to request the optical signal coupling / branching means and the optical signal multiplexing / demultiplexing means to have high demultiplexing characteristics, and the economical structure of the system can be realized.

[Brief description of drawings]

FIG. 1 is a principle of the present invention.

FIG. 2 is an embodiment of the present invention.

FIG. 3 is a second embodiment of the present invention.

4 is a waveform of each part of the configuration of FIG.

FIG. 5 is a configuration of an optical bidirectional transmission system.

FIG. 6 is a configuration of an optical multiplex transmission system.

FIG. 7 is a schematic diagram of a directional coupler or a wavelength division multiplexer.

FIG. 8 shows a case where an optical transmission line is cut in an optical bidirectional transmission system.

FIG. 9 shows a case where an optical cable is cut in an optical multiplex transmission system.

[Explanation of symbols]

 1 original signal (NRZ) 2 signal-1 (Return to Zero) 2 signal-2 (Return to One)

Claims (4)

[Claims] 1. A method for detecting an optical input break in an optical communication system for performing a plurality of communications using a single optical transmission line, comprising: A method for detecting an optical input interruption, which comprises converting the signal into a signal of 2 or 3), transmitting the signal, and determining an average level of a reception signal electrically converted in a receiver. 2. A method for detecting an optical input break in an optical communication system for performing a plurality of communications using a single optical transmission path, wherein an optical signal pre-modulated by a low frequency signal is modulated by a communication signal and transmitted. A method for detecting an optical input signal loss, which comprises detecting and determining a low frequency signal in a receiver. 3. An optical input disconnection detecting device according to the optical input disconnection detecting method according to claim 1, wherein in the receiver, an electrically converted main signal is applied to an average level detecting circuit, and an output of the average level detecting circuit is applied. An optical input break detection device characterized in that a voltage is compared with a reference voltage in a comparator. 4. An optical input disconnection detecting device according to the optical input disconnection detecting method according to claim 2, further comprising a main signal amplifier for amplifying an electrically converted signal, the main signal amplifier being connected to an output terminal of the main signal amplifier. In a receiver having a configuration for feeding back to the main signal amplifier via the automatic gain control circuit according to the peak value detected by the peak detection circuit, by connecting the superimposed signal determination circuit to the output of the peak detection circuit,
An optical input disconnection detection device characterized by detecting and determining a low frequency signal.