Loop-Type Optical Dataway System

The present invention relates to a loop-type optical dataway system.

A conventional loop-type optical dataway system has a double loop configuration as shown in FIG. 1.

Data transmission stations 13.sub.1, 13.sub.2, . . . , 13.sub.N are connected in optical fiber cables 11 and 12 to form the double loops. Each station 13.sub.i (i=1 through N) has optical receivers 21 and 22 for performing photoelectric conversion of light signals received from the optical fiber cables 11 and 12, and optical transmitters 31 and 32 for performing electrical/optical conversion of the input signals from the optical receivers 21 and 22 and producing the output light signals supplied to a following circuit through the optical fiber cables 11 and 12.

Data transmission is generally performed through one of the optical fiber cables 11 and 12. Assume that data transmission is being performed through the optical fiber cable 11. Then, a light signal supplied to the station 13.sub.i through the optical fiber cable 11 is converted into an electrical signal by the optical receiver 21. A converted output signal from the optical receiver 21 is supplied to a station controller (not shown) and to the optical transmitter 31. The optical transmitter 31 also receives an intra-office send signal from the station controller. The optical transmitter 31 converts the input signal into a light signal. The output light signal from the optical transmitter 31 is supplied to a following circuit through the optical fiber cable 11.

When a fault occurs in the optical fiber cables 11 and 12 or in the station 13i, the optical fiber cables 11 and 12 are connected before and after the fault section so as to form U-links (return links). Data transmission can be performed through the line excluding the fault section. FIG. 2 shows a system configuration after system reconfiguration when a fault occurs at the station 13.sub.1. In FIG. 2, U-links are formed at the stations 13.sub.N and 13.sub.2 before and after the station 13.sub.1. Thus, the fault station 13.sub.1 is isolated from the system, and data transmission between the stations 13.sub.2 through 13.sub.N can be performed through the optical fiber cables 11 and 12 (double loops).

In a double loop system as described above, U-links are formed to reconfigurate the system, thus allowing data transmission between stations after a fault occurs.

However, system reconfiguration in the prior art can only be performed when the fault occurs in a single station or in a single portion of the cables. Therefore, when two non-consecutive stations, for example, stations 13.sub.1 and 13.sub.4 are turned off for maintenance or modification, data transmission cannot be performed between the stations 13.sub.2 and 13.sub.3 and stations 13.sub.5 through 13.sub.N.

It is an object of the present invention to provide a loop-type optical dataway system which allows system reconfiguration even if a fault occurs at more than one station.

In order to achieve the above object of the present invention, there is provided a loop-type optical dataway system, comprising:

a single optical transmission line; and

a plurality of data transmission stations connected in a loop in said optical transmission line, each of said data transmission stations comprising:

a first optical directional coupler for distributing the light signal received from said optical transmission line into two directions;

an optical receiver for performing photoelectric conversion of one distributed-output light signal from said first optical directional coupler;

a station controller for receiving the output signal from said optical receiver and for producing an intra-office send signal;

a gate circuit for coupling the output signal from said optical receiver and the intra-office send signal;

an optical transmitter for performing electric/optical conversion of the output signal from said gate circuit; and

a second optical directional coupler for coupling the output light signal from said optical transmitter and the other distributed-output signal from said first optical directional coupler and for producing a coupled signal to a following station through said optical transmission line.

Other objects and features of the present invention will be apparent from the following description taken in connection with the accompanied drawings, in which:

FIG. 1 is a diagram showing a system configuration of a conventional loop-type optical dataway system;

FIG. 2 is a diagram showing an example of reconfiguration of the system shown in FIG. 1;

FIG. 3 is a diagram showing a loop-type optical dataway system according to an embodiment of the present invention;

FIG. 4 is a block diagram showing the configuration of a data transmission station shown in FIG. 3;

FIG. 5 is a diagram showing changes in output level for explaining the operation of the system shown in FIG. 3;

FIGS. 6A through 6C are waveform charts showing examples of waveforms of receive signals including repeater amplification signal components; and

FIG. 7 is a detailed block diagram of a level detector having an optical receiver 51 shown in FIG. 4.

FIG. 3 shows the system configuration of a loop-type optical dataway system according to an embodiment of the present invention. A plurality of data transfer stations 50.sub.1, 50.sub.2, . . . , 50.sub.N are connected in a loop to a single optical fiber cable 40. Each station 50.sub.i (i=1 through N) has an optical receiver 51 and an optical transmitter 52. The station 50.sub.i has a function of transmitting an input light signal received from the optical fiber cable 40 from its upstream side to the downstream side, and a function of repeating an input light signal from its upstream and supplying it to the downstream side using the optical receiver and transmitters 51 and 52.

The station 50.sub.i for realizing the above function has the configuration as shown in FIG. 4. The station 50.sub.i is an optical directional coupler (to be called hereinafter an optical coupler). Optical couplers 53 and 54 have terminals 53a through 53c and 54a through 54c. The terminal 53a of the optical coupler 53 is connected to the optical fiber cable 40. As a result, a light signal supplied to the terminal 53a of the optical coupler 53 through the optical fiber cable 40 is distributed to the terminals 53b and 53c. The optical receiver 51 is connected to the terminal 53c of the optical coupler 53. The optical receiver 51 converts and amplifies the light signal distributed from the terminal 53c into an electrical signal (receive signal) of a predetermined level. The optical receiver 51 has a level detector 70 for indicating if the light signal falls within a predetermined level range (e.g., within -20 dBm to -30 dBm). FIG. 7 shows the level detector 70. The light signal distributed from the output terminal 53c of the optical coupler 53 is subjected to photoelectric conversion by a photodiode 71. The electrical signal is amplified by an amplifier 72. The amplified signal is latched by a flip-flop 73 and is also supplied to an automatic gain control circuit (AGC circuit) 74. The AGC circuit 74 changes the gain in response to a change in an input signal from the amplifier 72 and produces a constant voltage (AGC voltage) V. The AGC voltage V is supplied to first and second voltage comparators 75 and 76. The first comparator 75 compares the AGC voltage with a first reference voltage VREF1 corresponding to the lowest output level (e.g., -20 dBm) of the light signal. When the AGC voltage exceeds the first reference voltage, the first comparator 75 produces a logic "1" signal. If the AGC voltage is below the first reference voltage, the first comparator 75 produces a logic "0" signal. Similarly, the second comparator 76 compares the AGC voltage with a second reference voltage VREF2 corresponding to the highest output level (e.g., -30 dBm) of the light signal. When the AGC voltage exceeds the second reference voltage, the second comparator 76 produces a logic "1" signal. When the AGC voltage is below the second reference voltage, the second comparator 76 produces a logic "0" signal. Output signals from the first and second comparators 75 and 76 are supplied to an exclusive-OR circuit 77. As a result, when the level of the light signal is within the range between -20 dBm and -30 dBm, the exclusive-OR circuit 77 produces a receive level signal 51a indicating that the light signal falls within a predetermined level range.

Referring to FIG. 4, a station controller 55 for performing data transfer control and a repeater gate 56 are connected to the optical receiver 51. An output signal (receive signal) from the optical receiver 51 is supplied to the station controller 55 and the repeater gate 56. The repeater gate 56 also receives the receive level signal 51a from the optical receiver 51. The repeater gate 56 comprises, for example, an AND gate. When the receive level signal 51a is high, i.e., when the light signal falls within the predetermined level range, the repeater gate 56 passes the output signal from the optical receiver 51.

An output signal from the repeater gate 56 and the send signal (intra-office send signal) from the station controller 55 are supplied to the OR gate 57. The gated signal by the OR gate 57 is supplied to the optical transmitter 52. The optical transmitter 52 also receives the receive level signal 51a from the optical receiver 51 and a send designation signal 55a from the station controller 55. The signal 55a is a control signal indicating the presence of the data transmission from the intra-office. The optical transmitter 52 has an output level varying function. Thus, when the signal 55a is logic "1," the optical transmitter 52 converts the output signal from the OR gate 57 into a light signal of a first level (e.g., 0 dBm). When the signal 51a is logic "1" and the signal 55a is logic "0," the optical transmitter 52 converts the output signal from the OR gate 57 into a light signal of a second level (e.g., -10 dBm).

The output of the optical transmitter 52 is connected to the terminal 54c of the optical coupler 54. The terminal 54b of the optical coupler 54 is connected to the terminal 53b of the optical coupler 53 through an optical fiber cable 58 as the internal transmission line. The terminal 54a of the optical coupler 54 is connected to the optical fiber cable 40. As a result, the light signal distributed from the terminal 53b of the coupler 53 is supplied to the terminal 54b of the optical coupler 54 through the cable 58, and the light signal from the optical transmitter 52 is supplied to the terminal 54c of the optical coupler 54. The optical coupler 54 couples the input signals at the terminals 54b and 54c and sends the coupled signal to a downstream station i+1 station through the optical fiber cable 40.

In this manner, according to this embodiment, the light signal transmitted from an upstream station is sent to a downstream station through the optical couplers 53 and 54. Therefore, even if the power supplies for a plurality of non-consecutive stations within the system are turned off, the light signal can be sent to a prescribed station. In addition, if the light signal supplied to the station 50.sub.i through the optical fiber cable 40 falls within a range between -20 dBm and -30 dBm, it is amplified to an output level of -10 dBm. Therefore, even if the light signal level is lowered through a fault section, such a level decrease can be conpensated for.

Assume that the station 50.sub.1 sends the light signal at an output level of 0 dBm. The light signal from the station 50.sub.1 is transmitted to the downstream station 50.sub.2 through the optical fiber cable 40. The level of the light signal from the station 50.sub.1 is gradually decreased during transmission to the station 50.sub.2, as shown in a diagram of FIG. 5 which shows changes in an output level (main signal path is also indicated by a thick line herein). The light signal level is further decreased by the optical coupler 53 of the station 50.sub.2, as indicated by symbol A in FIG. 5. At this time, if the light signal level is higher than -20 dBm as shown in FIG. 5, that is, if it is outside the receive level range X between -20 dBm and -30 dBm to be repeated, the light signal is not repeated by the station 50.sub.2, but instead simply passes through the optical coupler 54 and sent onto the optical fiber cable 40. The output level of the light signal is decreased as indicated by symbol B in FIG. 5 when it passes through the optical coupler 54.

The light signal passing through the station 50.sub.2 is tranmitted to the downstream station 50.sub.3 through the optical fiber cable 40 and is distributed and generated from the terminals 53b and 53c of the optical coupler 53 of the station 50.sub.3. When the output light signal level from the terminal 53c of the optical coupler 53 falls within the receive level range X to be repeated as shown in FIG. 5, the light signal is amplified up to an output level of -10 dBm and is transmitted to the downstream station 50.sub.4 through the optical fiber cable 40. When the output light signal level from the terminal 53c of the optical coupler 53 is below -30 dBm, the signal is ignored since it is below the receive level.

FIG. 6A shows a waveform of a passing signal component distributed from, for example, from the terminal 53b of the optical coupler 53 of the upstream station 50.sub.3. FIG. 6A shows a waveform of a repeated signal component from the optical transmitter 52 of the upstream station 50.sub.3. FIG. 6C shows a waveform of an output light signal (the passing signal component+the repeated signal component) from the upstream station 50.sub.3. In this case, the passing signal component shown in FIG. 6A can become a noise component of the receive signal. However, in this embodiment, the repeated output level is high, i.e., -10 dBm in comparison to the maximum value of the receive level range X to be repeated, i.e., -20 dBm. For this reason, a signal component 63 of the receive signal 60 is kept at a sufficient level, and the station 50.sub.i can normally receive data.

The above problem can undesirably occur between a send signal and a repeated signal. This occurs when, for example, the send signal from the station 50.sub.1 is repeated and returned by the station 50.sub.N in the transmission interval of the station 50.sub.1. In the case wherein a distance between the stations 50.sub.1 and 50.sub.N is small, if there is no difference between the signal level sent from the station 50.sub.1 to the station 50.sub.N and the send signal level of the station 50.sub.1, they cannot be discriminated. However, in this embodiment, since the send signal level is set at 0 dBm with respect to the repeating level of -10 dBm, this problem is resolved.