JPS591019B2 - Optical communication method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical communication system that allows highly reliable communication.

In conventional optical communication systems, a light emitting element such as a light emitting diode or a laser diode performs optical modulation according to an input signal, and the output light is transmitted through an optical transmission line consisting of an optical fiber. A light receiving element such as a photodiode or avalanche photodiode receives light and converts it into an electrical signal.

Therefore, the transmission distance largely depends on the output of the light emitting element, the transmission loss of the optical transmission line, the conversion efficiency of the light receiving element, the noise ratio, etc. Furthermore, if a failure occurs in the light emitting element or optical fiber on the transmitting side, communication will be completely impossible. The present invention uses an optical transmission line consisting of a pair of two optical fibers to equivalently improve reception sensitivity, and when a failure occurs in one of the two optical fibers. The objective is to provide a highly reliable optical communication system that enables reliable communication even when

Examples will be described in detail below. FIG. 1 is a diagram for explaining the basic configuration of a transmission system according to the present invention.
2 is a light emitting element such as a light emitting diode or a radiation diode, which is driven by transistors Q1 and Q2 that operate according to the input signal applied to the input terminal IN, and the light emission output is transmitted through an optical transmission line consisting of optical fibers 3 and 4. is sent to the receiving side. On the receiving side, light receiving elements 5 and 6 such as photodiodes, avalanche photodiodes, and phototransistors are provided corresponding to the optical fibers 3 and 4, and their output signals are applied to a differential amplifier T. Input signal is ``01''
When , the transistor Q1 is turned on and the light emitting element 1 emits light, and when the input signal is ``0'', the transistor Q2 is turned on and the light emitting element 2 emits light, so that the optical fibers 3 and 4 are connected to each other. In this case, mutually inverted optical signals are transmitted.

On the receiving side, for example, by configuring the output of the differential amplifier T to have a positive potential depending on the output of the light receiving element 5, and the output of the differential amplifier T to have a negative potential depending on the output of the light receiving element 6, the output At the terminal OUT, a received signal corresponding to the input signal and inverted to one positive or negative level is obtained. Therefore, the optical signal is equivalently doubled, and the S/N ratio can be improved. Furthermore, if a fault occurs in either one of the optical fibers 3, 4, the output of the differential amplifier 7 becomes an atmospheric potential, which can be detected and used for fault occurrence alarms, etc. As mentioned above, the transmission unit according to the present invention requires twice as many light emitting elements, light receiving elements, and optical fibers as in the conventional example, but it is possible to employ means such as arraying the light emitting elements and light receiving elements. Moreover, since optical fibers can be easily made into cables, the economical aspect is not so much of an issue, and the advantages such as improved S/N ratio are greater.

Note that since the optical signals are transmitted through the two optical fibers 3 and 4, it is necessary to adjust the output phase of the light receiving elements 5 and 6 if the transmission characteristics are not exactly the same. FIG. 2 is a block diagram of an embodiment of the present invention, in which 5, 6, and 7 are the light receiving elements and differential amplifiers shown in FIG. 1, and 8 and 9 are slicers that slice at the reference voltage VP level; 10,1
1 is a peak detector, 12 to 14 are differential amplifiers, VS,...
S? Reference voltages Q3 and Q4 are transistors for applying bias to the light receiving elements 3 and 4, and +V and -VB are voltages applied to the transistors Q3 and Q4. FIG. 3 is an explanatory diagram of the operation, and the signals a-1 of each part in FIG.
-1 is shown.

Further, T1 indicates a period of a normal state, and T2 indicates a period of an abnormal state in which one side of the signal is disconnected. In normal condition, the light receiving element 5
, 6 are opposite to each other, and therefore the output of the differential amplifier 7 is a pulse output that is inverted to positive and negative potentials. The positive potential signal of the output is sent to the slicer 8, and the negative potential signal is sent to the slicer 8. The signals A and b shown in FIG. 3A and b are obtained by slicing with the standard voltage using the isa 9, and the peaks of the signals A and b are detected by the peak detectors 10 and 11. ,b/)′S. ``6 if it is bi
Therefore, in the normal state, the output signals C and d of the peak detectors 10 and 11 are at the same level as within the period T1 of C and d in FIG. The output signal e becomes the atmosphere as shown in FIG.
The bias voltages H and i applied to the light receiving elements 5 and 6 are at the same level as shown in FIG. 3 H and i. Therefore, the sensitivities of the light receiving elements 5 and 6 become the same, and the output signal j of the differential amplifier 7 becomes a pulse signal that is inverted between positive and negative potentials, as shown in period T1 in FIG. 3j. If a failure occurs in the system of the light receiving element 6 and an abnormal state occurs in which the output of the light receiving element 6 is not applied to the differential amplifier 7, the output signal of the differential amplifier r may become a negative potential. Therefore, the output signal b of the slicer 9 is continuously "0".
”, therefore, the output signal Dii6 of the peak detector 11
O'', the output signal e of the differential amplifier 12 becomes a certain level as shown in period T2 in FIG. 3e, and the output signal g of the differential amplifier 14 decreases.

As a result, the bias voltage h of the light receiving element 5 increases to the negative side and its sensitivity increases, and the bias voltage 1 of the light receiving element 6 decreases to the negative side and its sensitivity decreases. That is, the sensitivity of the light-receiving element on the side where the optical signal is cut off is reduced, the sensitivity of the light-receiving element on the other side is increased, and the output signal j of the differential amplifier 7 becomes as shown in period T2 in FIG. 3j. This results in a pulse signal with a high level. Even if the signal on one side is cut off as described above, the level of the output signal j of the differential amplifier 7 increases, so that optical communication can be continued.

As explained above, the present invention operates in the same way as a normal automatic gain control (AGC) system during normal operation, and controls each transmission system to have the same signal level (absolute value). In the event of an abnormality, the sensitivity of the faulty transmission system is reduced sufficiently to the extent that the signal level does not appear as an output, and conversely, the sensitivity of the normal transmission system is increased to a signal level that allows communication to continue. . The amount of increase in sensitivity of this normal transmission system is approximately twice as much. In addition, when the conventional automatic gain control system works to compensate for a low optical signal due to a failure, it abnormally increases the bias voltage of the light receiving element to the negative side, causing the light receiving element to This is extremely advantageous in preventing the device from exceeding the allowable voltage value and causing device deterioration.

In addition, even if one of the two optical fibers is disconnected due to a fault, the optical signal can be transmitted using the other system, making it highly reliable, and making it possible to transmit optical signals using the other system. This configuration has the advantage that it is easy to improve the S/N ratio during normal operation and detect the occurrence of a failure.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the basic configuration of a transmission system according to the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, and FIG. 3 is an explanatory diagram of its motion. 1 and 2 are light emitting elements, 3 and 4 are optical fibers, 5 and 6 are light receiving elements, 7 is a differential amplifier, 8 and 9 are slicers, 10 and 11
is a peak detector, and 12 to 14 are differential amplifiers.

Claims (1)

[Claims] 1 The optical transmission line is composed of two optical fibers, and each optical fiber simultaneously transmits mutually inverted optical signals. On the receiving side, a light receiving element is provided for each optical fiber, and the received signal is photoelectrically converted by the light receiving element. Amplify and output the level difference with a differential amplifier, monitor the positive and negative levels of the output signal of the differential amplifier, and detect a continuous positive or negative level to detect a failure in the optical transmission line. , controlling the bias voltage of the light-receiving element corresponding to the optical fiber in which the failure has occurred to reduce the sensitivity of the light-receiving element to such an extent that level fluctuations in the received signal do not appear as output;
An optical communication system characterized in that the bias voltage of a light receiving element corresponding to a normal optical fiber is controlled to increase the sensitivity of the light receiving element to about twice that of a normal optical fiber.