| 摘要 |
An optical coupling unit connected to a single optical fiber includes first to fourth directional couplers each having three terminals. The optical coupling unit receives at a first side a first optical signal transmitted in a first direction through the fiber, and at a second side a second optical signal transmitted in a second direction through the fiber. The first optical signal is guided to the second side through first and second couplers and is guided through the first and third couplers to a first optical receiver. The second optical signal is guided to the first side through the second and first couplers and is guided to a second optical receiver through the second and fourth couplers. When an optical signal input to the second optical receiver is below a predetermined level, a first optical transmitter produces an optical signal formed by O-ring a second electrical signal from the second optical receiver and a transmission signal. The optical signal from the first optical transmitter is guided to the first side through the third and first couplers. When an optical signal input to the first optical receiver is below a predetermined level, an optical signal corresponding to the signal formed by ORing the first electrical signal from the first optical receiver and the transmission signal is output from a second optical transmitter and is guided through the fourth and second couplers to the second side. The signal obtained by ORing the first and second electrical signals is used as reception signal.
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