Communication device and communication system

A communication device comprising:

• a transmission unit configured to transmit an optical signal;
• a reception unit configured to receive an optical signal;
• a transmission port;
• a reception port;
• a blocking unit connected to the transmission unit;
• and
• an optical coupler connected to the blocking unit, the reception unit, the transmission port, and the reception port,
• wherein the blocking unit is configured to allow the optical signal transmitted from the transmission unit to pass therethrough, but block an optical signal that travels toward the transmission unit, and
• the optical coupler is configured to output the optical signal transmitted from the transmission unit to each of the transmission port and the reception port, output an optical signal transmitted from the reception port to each of the reception unit and the blocking unit, and output the optical signal transmitted from the transmission port to each of the reception unit and the blocking unit.